diff --git a/lib/config/definitions.js b/lib/config/definitions.js
index 99ef31037746573a0a9183f26d685ed7db144214..c518f6a5877b9105359d4ccc163098d41040634c 100644
--- a/lib/config/definitions.js
+++ b/lib/config/definitions.js
@@ -154,6 +154,7 @@ const options = [
     description: 'String copy of npmrc file. Use \\n instead of line breaks',
     stage: 'branch',
     type: 'string',
+    mergeable: 'true',
   },
   {
     name: 'yarnrc',
diff --git a/lib/config/index.js b/lib/config/index.js
index 97c850a381c6df6e88057f5b23d716bd1a049005..60ca1a110c1e9ceb1271291990dd3dccebf2a4de 100644
--- a/lib/config/index.js
+++ b/lib/config/index.js
@@ -126,6 +126,8 @@ function mergeChildConfig(parentConfig, childConfig) {
         config[option.name] = (parentConfig[option.name] || []).concat(
           config[option.name] || []
         );
+      } else if (option.type === 'string') {
+        config[option.name] = parentConfig[option.name] + config[option.name];
       } else {
         config[option.name] = {
           ...parentConfig[option.name],
diff --git a/test/config/__snapshots__/index.spec.js.snap b/test/config/__snapshots__/index.spec.js.snap
index 73df5d60dca5903b72ce29704bc547a9a64c6cc9..56c31694d8c4b02c2a60b8953f2c7438bd966b87 100644
--- a/test/config/__snapshots__/index.spec.js.snap
+++ b/test/config/__snapshots__/index.spec.js.snap
@@ -1,5 +1,12 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`config/index mergeChildConfig(parentConfig, childConfig) handles mergeable strings 1`] = `
+"_auth=abcd-abcd-abcd-abcd
+always-auth=true
+registry=some-registry
+"
+`;
+
 exports[`config/index mergeChildConfig(parentConfig, childConfig) merges 1`] = `
 Object {
   "branchName": "{{branchPrefix}}lock-file-maintenance",
diff --git a/test/config/index.spec.js b/test/config/index.spec.js
index 582a3cddd9fa06f0ae02a854ac704a5acfcf08b3..87ddd5924f58f073b35b984553eb33f7ccc3dbbb 100644
--- a/test/config/index.spec.js
+++ b/test/config/index.spec.js
@@ -235,5 +235,15 @@ describe('config/index', () => {
       const config = configParser.mergeChildConfig(parentConfig, undefined);
       expect(config).toMatchObject(parentConfig);
     });
+    it('handles mergeable strings', () => {
+      const parentConfig = { ...defaultConfig };
+      parentConfig.npmrc = '_auth=abcd-abcd-abcd-abcd\n';
+      const childConfig = {
+        npmrc: 'always-auth=true\nregistry=some-registry\n',
+      };
+      const configParser = require('../../lib/config/index.js');
+      const config = configParser.mergeChildConfig(parentConfig, childConfig);
+      expect(config.npmrc).toMatchSnapshot();
+    });
   });
 });