diff --git a/packages/plugin/src/message-type-extensions/well-known-types.ts b/packages/plugin/src/message-type-extensions/well-known-types.ts
index 673b921e..007e046a 100644
--- a/packages/plugin/src/message-type-extensions/well-known-types.ts
+++ b/packages/plugin/src/message-type-extensions/well-known-types.ts
@@ -358,8 +358,9 @@ export class WellKnownTypes implements CustomMethodGenerator {
              * Encode \`${descriptor.name}\` to JSON object. 
              */
             function internalJsonWrite(message: ${FieldMask}, options: ${JsonWriteOptions}): ${JsonValue} {
+                const invalidFieldMaskJsonRegex = /[A-Z]|(_([.0-9_]|$))/g;
                 return message.paths.map(p => {
-                    if (p.match(/_[0-9]?_/g) || p.match(/[A-Z]/g))
+                    if (invalidFieldMaskJsonRegex.test(p))
                         throw new Error("Unable to encode FieldMask to JSON. lowerCamelCase of path name \\""+p+"\\" is irreversible.");
                     return ${lowerCamelCase}(p);
                 }).join(",");
@@ -379,7 +380,7 @@ export class WellKnownTypes implements CustomMethodGenerator {
                     if (str.includes('_'))
                         throw new Error("Unable to parse FieldMask from JSON. Path names must be lowerCamelCase.");
                     let sc = str.replace(/[A-Z]/g, letter => "_" + letter.toLowerCase());
-                    return (sc[0] === "_") ? sc.substring(1) : sc;
+                    return sc;
                 };
                 target.paths = json.split(",").map(camelToSnake);
                 return target;
diff --git a/packages/test-generated/spec/google.protobuf.field_mask.spec.ts b/packages/test-generated/spec/google.protobuf.field_mask.spec.ts
index b6450f4f..cc5ef84b 100644
--- a/packages/test-generated/spec/google.protobuf.field_mask.spec.ts
+++ b/packages/test-generated/spec/google.protobuf.field_mask.spec.ts
@@ -2,8 +2,27 @@ import {FieldMask} from "../ts-out/google/protobuf/field_mask";
 
 describe('google.protobuf.FieldMask', function () {
 
-
     describe('toJson()', function () {
+        // Based on https://github.com/protocolbuffers/protobuf/blob/e8ae137c96444ea313485ed1118c5e43b2099cf1/src/google/protobuf/util/field_mask_util_test.cc#L69-L82
+        it('converts snake_case to camelCase', function () {
+            let mask: FieldMask = { paths: [] };
+            function snakeToCamel(s: string) {
+                mask.paths[0] = s;
+                return FieldMask.toJson(mask);
+            }
+            expect(snakeToCamel('foo_bar')).toBe('fooBar');
+            expect(snakeToCamel('_foo_bar')).toBe('FooBar');
+            expect(snakeToCamel('foo3_bar')).toBe('foo3Bar');
+            // No uppercase letter is allowed.
+            expect(() => snakeToCamel('Foo')).toThrowError();
+            // Any character after a "_" must be a lowercase letter.
+            //   1. "_" cannot be followed by another "_".
+            //   2. "_" cannot be followed by a digit.
+            //   3. "_" cannot appear as the last character.
+            expect(() => snakeToCamel('foo__bar')).toThrowError();
+            expect(() => snakeToCamel('foo_3bar')).toThrowError();
+            expect(() => snakeToCamel('foo_bar_')).toThrowError();
+        });
         it('returns expected JSON', function () {
             let mask: FieldMask = {
                 paths: [
@@ -29,6 +48,17 @@ describe('google.protobuf.FieldMask', function () {
     });
 
     describe('fromJson()', function () {
+        // Based on https://github.com/protocolbuffers/protobuf/blob/e8ae137c96444ea313485ed1118c5e43b2099cf1/src/google/protobuf/util/field_mask_util_test.cc#L84-L90
+        it('converts camelCase to snake_case', function () {
+            function camelToSnake(s: string) {
+                return FieldMask.fromJson(s).paths[0];
+            }
+            expect(camelToSnake('fooBar')).toBe('foo_bar');
+            expect(camelToSnake('FooBar')).toBe('_foo_bar');
+            expect(camelToSnake('foo3Bar')).toBe('foo3_bar');
+            // "_"s are not allowed.
+            expect(() => camelToSnake('foo_bar')).toThrowError();
+        });
         it('parses JSON as expected', function () {
             let expected: FieldMask = {
                 paths: [