diff --git a/CHANGELOG.md b/CHANGELOG.md
index ef2f8070b1019..3e3c6959c55d7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -162,6 +162,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 ### Fixed
 
+- Fixed `apply_to_collection` works on Custom Collections now ([#7851](https://github.com/PyTorchLightning/pytorch-lightning/pull/7851))
+
 - Fixed ambiguous warning when both overfit and train dataloader shuffling are enabled ([#7685](https://github.com/PyTorchLightning/pytorch-lightning/pull/7685))
 
 - Fixed dataloaders are not reset when tuning the model ([#7566](https://github.com/PyTorchLightning/pytorch-lightning/pull/7566))
diff --git a/pytorch_lightning/core/lightning.py b/pytorch_lightning/core/lightning.py
index a24dc9d367f87..67dceb37579fc 100644
--- a/pytorch_lightning/core/lightning.py
+++ b/pytorch_lightning/core/lightning.py
@@ -26,7 +26,7 @@
 from argparse import Namespace
 from functools import partial
 from pathlib import Path
-from typing import Any, Callable, Dict, List, Optional, Sequence, Tuple, TYPE_CHECKING, Union
+from typing import Any, Callable, Dict, List, Mapping, Optional, Sequence, Tuple, TYPE_CHECKING, Union
 
 import torch
 from torch import ScriptModule, Tensor
@@ -363,7 +363,7 @@ def log(
 
     def log_dict(
         self,
-        dictionary: Dict[str, _METRIC_COLLECTION],
+        dictionary: Mapping[str, _METRIC_COLLECTION],
         prog_bar: bool = False,
         logger: bool = True,
         on_step: Optional[bool] = None,
diff --git a/pytorch_lightning/utilities/apply_func.py b/pytorch_lightning/utilities/apply_func.py
index 456bc5b5cd406..70d8e4a2b6ff6 100644
--- a/pytorch_lightning/utilities/apply_func.py
+++ b/pytorch_lightning/utilities/apply_func.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 import operator
 from abc import ABC
+from collections import OrderedDict
 from collections.abc import Mapping, Sequence
 from copy import copy
 from functools import partial
@@ -92,12 +93,12 @@ def apply_to_collection(
 
     # Recursively apply to collection items
     if isinstance(data, Mapping):
-        out = []  # can't use dict, need to preserve order if `OrderedDict`
+        out = []
         for k, v in data.items():
             v = apply_to_collection(v, dtype, function, *args, wrong_dtype=wrong_dtype, **kwargs)
             if include_none or v is not None:
                 out.append((k, v))
-        return elem_type(out)
+        return elem_type(OrderedDict(out))
 
     is_namedtuple = _is_namedtuple(data)
     is_sequence = isinstance(data, Sequence) and not isinstance(data, str)
diff --git a/tests/utilities/test_apply_func.py b/tests/utilities/test_apply_func.py
index 1cc6ded7b25db..2457cf998c2cd 100644
--- a/tests/utilities/test_apply_func.py
+++ b/tests/utilities/test_apply_func.py
@@ -84,6 +84,16 @@ def test_recursive_application_to_collection():
     reduced = apply_to_collection(OrderedDict([('b', 2), ('a', 1)]), int, lambda x: str(x))
     assert reduced == OrderedDict([('b', '2'), ('a', '1')])
 
+    # custom mappings
+    class _CustomCollection(dict):
+
+        def __init__(self, initial_dict):
+            super().__init__(initial_dict)
+
+    to_reduce = _CustomCollection({'a': 1, 'b': 2, 'c': 3})
+    reduced = apply_to_collection(to_reduce, int, lambda x: str(x))
+    assert reduced == _CustomCollection({'a': '1', 'b': '2', 'c': '3'})
+
 
 def test_apply_to_collection_include_none():
     to_reduce = [1, 2, 3.4, 5.6, 7]