diff --git a/pandas_gbq/gbq.py b/pandas_gbq/gbq.py
index 9c02538d..4ec7f804 100644
--- a/pandas_gbq/gbq.py
+++ b/pandas_gbq/gbq.py
@@ -13,6 +13,7 @@
     bigquery_storage_v1beta1 = None
 
 from pandas_gbq.exceptions import AccessDenied
+import pandas_gbq.schema
 
 logger = logging.getLogger(__name__)
 
@@ -1269,12 +1270,7 @@ def create(self, table_id, schema):
         table_ref = self.client.dataset(self.dataset_id).table(table_id)
         table = Table(table_ref)
 
-        # Manually create the schema objects, adding NULLABLE mode
-        # as a workaround for
-        # https://github.com/GoogleCloudPlatform/google-cloud-python/issues/4456
-        for field in schema["fields"]:
-            if "mode" not in field:
-                field["mode"] = "NULLABLE"
+        schema = pandas_gbq.schema.add_default_nullable_mode(schema)
 
         table.schema = [
             SchemaField.from_api_repr(field) for field in schema["fields"]
diff --git a/pandas_gbq/load.py b/pandas_gbq/load.py
index cb190f86..3e9d570e 100644
--- a/pandas_gbq/load.py
+++ b/pandas_gbq/load.py
@@ -66,12 +66,7 @@ def load_chunks(
     if schema is None:
         schema = pandas_gbq.schema.generate_bq_schema(dataframe)
 
-    # Manually create the schema objects, adding NULLABLE mode
-    # as a workaround for
-    # https://github.com/GoogleCloudPlatform/google-cloud-python/issues/4456
-    for field in schema["fields"]:
-        if "mode" not in field:
-            field["mode"] = "NULLABLE"
+    schema = pandas_gbq.schema.add_default_nullable_mode(schema)
 
     job_config.schema = [
         bigquery.SchemaField.from_api_repr(field) for field in schema["fields"]
diff --git a/pandas_gbq/schema.py b/pandas_gbq/schema.py
index 91963b7c..bb18fabc 100644
--- a/pandas_gbq/schema.py
+++ b/pandas_gbq/schema.py
@@ -1,5 +1,7 @@
 """Helper methods for BigQuery schemas"""
 
+import copy
+
 
 def generate_bq_schema(dataframe, default_type="STRING"):
     """Given a passed dataframe, generate the associated Google BigQuery schema.
@@ -62,3 +64,16 @@ def update_schema(schema_old, schema_new):
             output_fields.append(field)
 
     return {"fields": output_fields}
+
+
+def add_default_nullable_mode(schema):
+    """Manually create the schema objects, adding NULLABLE mode."""
+    # Workaround for:
+    # https://github.com/GoogleCloudPlatform/google-cloud-python/issues/4456
+    #
+    # Returns a copy rather than modifying the mutable arg,
+    # per Issue #277
+    result = copy.deepcopy(schema)
+    for field in result["fields"]:
+        field.setdefault("mode", "NULLABLE")
+    return result
diff --git a/tests/unit/test_gbq.py b/tests/unit/test_gbq.py
index bf609385..f2e52462 100644
--- a/tests/unit/test_gbq.py
+++ b/tests/unit/test_gbq.py
@@ -1,5 +1,7 @@
 # -*- coding: utf-8 -*-
 
+import copy
+import datetime
 from unittest import mock
 
 import numpy
@@ -477,3 +479,45 @@ def test_generate_bq_schema_deprecated():
     with pytest.warns(FutureWarning):
         df = DataFrame([[1, "two"], [3, "four"]])
         gbq.generate_bq_schema(df)
+
+
+def test_load_does_not_modify_schema_arg():
+    # Test of Issue # 277
+    df = DataFrame(
+        {
+            "field1": ["a", "b"],
+            "field2": [1, 2],
+            "field3": [datetime.date(2019, 1, 1), datetime.date(2019, 5, 1)],
+        }
+    )
+    original_schema = [
+        {"name": "field1", "type": "STRING", "mode": "REQUIRED"},
+        {"name": "field2", "type": "INTEGER"},
+        {"name": "field3", "type": "DATE"},
+    ]
+    original_schema_cp = copy.deepcopy(original_schema)
+    gbq.to_gbq(
+        df,
+        "dataset.schematest",
+        project_id="my-project",
+        table_schema=original_schema,
+        if_exists="fail",
+    )
+    assert original_schema == original_schema_cp
+
+    # Test again now that table exists - behavior will differ internally
+    # branch at if table.exists(table_id)
+    original_schema = [
+        {"name": "field1", "type": "STRING", "mode": "REQUIRED"},
+        {"name": "field2", "type": "INTEGER"},
+        {"name": "field3", "type": "DATE"},
+    ]
+    original_schema_cp = copy.deepcopy(original_schema)
+    gbq.to_gbq(
+        df,
+        "dataset.schematest",
+        project_id="my-project",
+        table_schema=original_schema,
+        if_exists="append",
+    )
+    assert original_schema == original_schema_cp