diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1a69e035d..706339808 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,7 +6,7 @@
 
 ### Breaking Changes
 
-_None_
+- `EnvManager`: populate the process `ENV` from the loaded `.env` file by default (no-override semantics — pre-existing `ENV` values win), so fastlane actions that read their `default_value:` from `ENV` find loaded secrets without callers having to thread them through explicitly. Pass `mutate_env: false` to opt out and keep the parse-only / instance-isolation semantics introduced in [#578]. `EnvManager.reset!` now also rolls back the keys it added. [#XXX]
 
 ### New Features
 
diff --git a/lib/fastlane/plugin/wpmreleasetoolkit/env_manager/env_manager.rb b/lib/fastlane/plugin/wpmreleasetoolkit/env_manager/env_manager.rb
index 75078af5d..2d820cbcd 100644
--- a/lib/fastlane/plugin/wpmreleasetoolkit/env_manager/env_manager.rb
+++ b/lib/fastlane/plugin/wpmreleasetoolkit/env_manager/env_manager.rb
@@ -15,10 +15,22 @@ class << self
       end
 
       # Set up by loading the .env file with the given name.
+      #
+      # When `mutate_env` is true (the default), values from the .env file are
+      # layered into the process `ENV` using no-override semantics: keys already
+      # set in `ENV` (e.g. by CI) win. This lets fastlane actions that look up
+      # values via `ENV.fetch(...)` for their `default_value:` find them without
+      # the caller having to thread them through explicitly.
+      #
+      # Pass `mutate_env: false` to keep `ENV` pristine — values are still
+      # accessible via `get_required_env!`, but only through this instance.
+      # Useful for tests that want isolation, or for callers that prefer to
+      # control `ENV` themselves.
       def initialize(
         env_file_name:,
         env_file_folder: File.join(Dir.home, '.a8c-apps'),
         example_env_file_path: 'fastlane/example.env',
+        mutate_env: true,
         print_error_lambda: ->(message) { FastlaneCore::UI.user_error!(message) },
         print_warning_lambda: ->(message) { FastlaneCore::UI.important(message) }
       )
@@ -31,10 +43,34 @@ def initialize(
           @print_warning_lambda.call("Warning: env file not found at #{@env_path}. Environment variables may not be loaded.")
         end
 
-        # Parse rather than load so we don't mutate the global ENV. Each instance
-        # gets its own view, and values from the process environment (e.g. set by
-        # CI) still take precedence — see `env_value`.
+        # Always parse rather than load: it lets us track exactly which keys we
+        # add to `ENV` so `reset!` can undo only those, without disturbing keys
+        # that pre-existed in the process environment.
         @loaded_env = File.exist?(@env_path) ? Dotenv.parse(@env_path) : {}
+        # Records the value this instance wrote for each key it added. On
+        # restore, we delete only if `ENV[key]` still matches — if a later
+        # caller overwrote it, their value is left alone.
+        @mutations = {}
+
+        return unless mutate_env
+
+        @loaded_env.each do |key, value|
+          # No-override: anything already in `ENV` (e.g. set by CI) wins.
+          next if ENV.key?(key)
+
+          ENV[key] = value
+          @mutations[key] = value
+        end
+      end
+
+      # Remove from `ENV` any keys this instance added via `mutate_env: true`,
+      # but only if the value is still the one we wrote. Keys that a later
+      # caller has overwritten are left untouched.
+      # Idempotent — calling more than once is safe. Used by `reset!` and
+      # available for callers that want to roll back manually.
+      def restore_env!
+        @mutations.each { |key, value| ENV.delete(key) if ENV[key] == value }
+        @mutations = {}
       end
 
       # Use this instead of getting values from `ENV` directly. It will throw an error if the requested value is missing or empty.
@@ -135,8 +171,10 @@ def self.require_env_vars!(*keys)
         default!.require_env_vars!(*keys)
       end
 
-      # Clears the default instance, useful for test teardown.
+      # Clears the default instance, useful for test teardown. Also rolls back
+      # any `ENV` mutations the default instance performed via `mutate_env`.
       def self.reset!
+        @default&.restore_env!
         @default = nil
       end
 
diff --git a/spec/env_manager_spec.rb b/spec/env_manager_spec.rb
index 11ca45375..413e2b3d8 100644
--- a/spec/env_manager_spec.rb
+++ b/spec/env_manager_spec.rb
@@ -62,7 +62,7 @@
       expect(manager.env_example_path).to eq('fastlane/example.env')
     end
 
-    it 'loads values from the .env file without mutating ENV' do
+    it 'loads values from the .env file into ENV by default' do
       ENV.delete('TEST_INIT_VAR')
 
       with_tmp_file(named: 'test.env', content: "TEST_INIT_VAR=loaded\n") do |path|
@@ -72,22 +72,40 @@
           print_error_lambda: print_error_lambda
         )
 
+        expect(manager.get_required_env!('TEST_INIT_VAR')).to eq('loaded')
+        expect(ENV.fetch('TEST_INIT_VAR', nil)).to eq('loaded')
+      end
+    end
+
+    it 'leaves ENV untouched when mutate_env is false' do
+      ENV.delete('TEST_INIT_VAR')
+
+      with_tmp_file(named: 'test.env', content: "TEST_INIT_VAR=loaded\n") do |path|
+        manager = described_class.new(
+          env_file_name: File.basename(path),
+          env_file_folder: File.dirname(path),
+          mutate_env: false,
+          print_error_lambda: print_error_lambda
+        )
+
         expect(manager.get_required_env!('TEST_INIT_VAR')).to eq('loaded')
         expect(ENV.fetch('TEST_INIT_VAR', nil)).to be_nil
       end
     end
 
-    it 'keeps multiple instances isolated from each other' do
+    it 'keeps multiple instances isolated when mutate_env is false' do
       with_tmp_file(named: 'a.env', content: "SHARED_KEY=from_a\n") do |path_a|
         with_tmp_file(named: 'b.env', content: "SHARED_KEY=from_b\n") do |path_b|
           manager_a = described_class.new(
             env_file_name: File.basename(path_a),
             env_file_folder: File.dirname(path_a),
+            mutate_env: false,
             print_error_lambda: print_error_lambda
           )
           manager_b = described_class.new(
             env_file_name: File.basename(path_b),
             env_file_folder: File.dirname(path_b),
+            mutate_env: false,
             print_error_lambda: print_error_lambda
           )
 
@@ -108,6 +126,31 @@
         )
 
         expect(manager.get_required_env!('PRECEDENCE_KEY')).to eq('from_env')
+        # No-override: the pre-existing ENV value is left in place.
+        expect(ENV.fetch('PRECEDENCE_KEY')).to eq('from_env')
+      end
+    end
+
+    it 'lets the first instance win when multiple mutate_env instances share a key' do
+      ENV.delete('FIRST_WINS_KEY')
+
+      with_tmp_file(named: 'a.env', content: "FIRST_WINS_KEY=from_a\n") do |path_a|
+        with_tmp_file(named: 'b.env', content: "FIRST_WINS_KEY=from_b\n") do |path_b|
+          described_class.new(
+            env_file_name: File.basename(path_a),
+            env_file_folder: File.dirname(path_a),
+            print_error_lambda: print_error_lambda
+          )
+          described_class.new(
+            env_file_name: File.basename(path_b),
+            env_file_folder: File.dirname(path_b),
+            print_error_lambda: print_error_lambda
+          )
+
+          # The second instance's value never lands in ENV because the first
+          # already populated the key.
+          expect(ENV.fetch('FIRST_WINS_KEY')).to eq('from_a')
+        end
       end
     end
 
@@ -329,6 +372,85 @@
     end
   end
 
+  describe '#restore_env!' do
+    it 'removes only the keys this instance added to ENV' do
+      ENV.delete('RESTORE_NEW_KEY')
+      ENV['RESTORE_PREEXISTING_KEY'] = 'original'
+
+      with_tmp_file(
+        named: 'restore.env',
+        content: "RESTORE_NEW_KEY=loaded\nRESTORE_PREEXISTING_KEY=from_file\n"
+      ) do |path|
+        manager = described_class.new(
+          env_file_name: File.basename(path),
+          env_file_folder: File.dirname(path),
+          print_error_lambda: print_error_lambda
+        )
+
+        expect(ENV.fetch('RESTORE_NEW_KEY')).to eq('loaded')
+        expect(ENV.fetch('RESTORE_PREEXISTING_KEY')).to eq('original')
+
+        manager.restore_env!
+
+        expect(ENV.fetch('RESTORE_NEW_KEY', nil)).to be_nil
+        expect(ENV.fetch('RESTORE_PREEXISTING_KEY')).to eq('original')
+      end
+    end
+
+    it 'is idempotent' do
+      ENV.delete('IDEMPOTENT_KEY')
+
+      with_tmp_file(named: 'restore.env', content: "IDEMPOTENT_KEY=loaded\n") do |path|
+        manager = described_class.new(
+          env_file_name: File.basename(path),
+          env_file_folder: File.dirname(path),
+          print_error_lambda: print_error_lambda
+        )
+
+        manager.restore_env!
+        expect { manager.restore_env! }.not_to raise_error
+        expect(ENV.fetch('IDEMPOTENT_KEY', nil)).to be_nil
+      end
+    end
+
+    it 'leaves a key alone when a later caller has overwritten it' do
+      ENV.delete('RESTORE_OVERWRITTEN_KEY')
+
+      with_tmp_file(named: 'restore.env', content: "RESTORE_OVERWRITTEN_KEY=loaded\n") do |path|
+        manager = described_class.new(
+          env_file_name: File.basename(path),
+          env_file_folder: File.dirname(path),
+          print_error_lambda: print_error_lambda
+        )
+
+        expect(ENV.fetch('RESTORE_OVERWRITTEN_KEY')).to eq('loaded')
+
+        # Simulate a later caller overwriting the value the manager set.
+        ENV['RESTORE_OVERWRITTEN_KEY'] = 'overwritten'
+
+        manager.restore_env!
+
+        expect(ENV.fetch('RESTORE_OVERWRITTEN_KEY')).to eq('overwritten')
+      end
+    end
+
+    it 'is a no-op when mutate_env was false' do
+      ENV.delete('NOOP_KEY')
+
+      with_tmp_file(named: 'restore.env', content: "NOOP_KEY=loaded\n") do |path|
+        manager = described_class.new(
+          env_file_name: File.basename(path),
+          env_file_folder: File.dirname(path),
+          mutate_env: false,
+          print_error_lambda: print_error_lambda
+        )
+
+        expect { manager.restore_env! }.not_to raise_error
+        expect(ENV.fetch('NOOP_KEY', nil)).to be_nil
+      end
+    end
+  end
+
   describe 'CI environment helpers' do
     subject(:manager) do
       described_class.new(
@@ -522,6 +644,42 @@
 
         expect(described_class).not_to be_configured
       end
+
+      it 'rolls back ENV mutations the default instance performed' do
+        described_class.reset!
+        ENV.delete('RESET_ROLLBACK_KEY')
+
+        with_tmp_file(named: 'reset.env', content: "RESET_ROLLBACK_KEY=loaded\n") do |path|
+          described_class.set_up(
+            env_file_name: File.basename(path),
+            env_file_folder: File.dirname(path),
+            print_error_lambda: print_error_lambda
+          )
+
+          expect(ENV.fetch('RESET_ROLLBACK_KEY')).to eq('loaded')
+
+          described_class.reset!
+
+          expect(ENV.fetch('RESET_ROLLBACK_KEY', nil)).to be_nil
+        end
+      end
+
+      it 'leaves pre-existing ENV entries alone on rollback' do
+        described_class.reset!
+        ENV['PREEXISTING_KEY'] = 'original'
+
+        with_tmp_file(named: 'reset.env', content: "PREEXISTING_KEY=from_file\n") do |path|
+          described_class.set_up(
+            env_file_name: File.basename(path),
+            env_file_folder: File.dirname(path),
+            print_error_lambda: print_error_lambda
+          )
+
+          described_class.reset!
+
+          expect(ENV.fetch('PREEXISTING_KEY')).to eq('original')
+        end
+      end
     end
 
     describe '.configured?' do