diff --git a/.changes/unreleased/Under the Hood-20230817-134548.yaml b/.changes/unreleased/Under the Hood-20230817-134548.yaml
new file mode 100644
index 00000000000..82e445d80e9
--- /dev/null
+++ b/.changes/unreleased/Under the Hood-20230817-134548.yaml	
@@ -0,0 +1,6 @@
+kind: Under the Hood
+body: Fix mypy warnings for ManifestLoader.load()
+time: 2023-08-17T13:45:48.937252-04:00
+custom:
+  Author: gshank
+  Issue: "8401"
diff --git a/core/dbt/parser/manifest.py b/core/dbt/parser/manifest.py
index a4bcef28f01..9fded512e42 100644
--- a/core/dbt/parser/manifest.py
+++ b/core/dbt/parser/manifest.py
@@ -79,6 +79,7 @@
     load_source_file,
     FileDiff,
     ReadFilesFromDiff,
+    ReadFiles,
 )
 from dbt.parser.partial import PartialParsing, special_override_macros
 from dbt.contracts.graph.manifest import (
@@ -259,7 +260,7 @@ def __init__(
         # We need to know if we're actually partially parsing. It could
         # have been enabled, but not happening because of some issue.
         self.partially_parsing = False
-        self.partial_parser = None
+        self.partial_parser: Optional[PartialParsing] = None
 
         # This is a saved manifest from a previous run that's used for partial parsing
         self.saved_manifest: Optional[Manifest] = self.read_manifest_for_partial_parse()
@@ -331,7 +332,7 @@ def get_full_manifest(
         return manifest
 
     # This is where the main action happens
-    def load(self):
+    def load(self) -> Manifest:
         start_read_files = time.perf_counter()
 
         # This updates the "files" dictionary in self.manifest, and creates
@@ -340,6 +341,7 @@ def load(self):
         # of parsers to lists of file strings. The file strings are
         # used to get the SourceFiles from the manifest files.
         saved_files = self.saved_manifest.files if self.saved_manifest else {}
+        file_reader: Optional[ReadFiles] = None
         if self.file_diff:
             # We're getting files from a file diff
             file_reader = ReadFilesFromDiff(
@@ -403,7 +405,7 @@ def load(self):
                     }
 
                     # get file info for local logs
-                    parse_file_type = None
+                    parse_file_type: str = ""
                     file_id = self.partial_parser.processing_file
                     if file_id:
                         source_file = None
@@ -484,7 +486,7 @@ def load(self):
             self.manifest.rebuild_disabled_lookup()
 
             # Load yaml files
-            parser_types = [SchemaParser]
+            parser_types = [SchemaParser]  # type: ignore
             for project in self.all_projects.values():
                 if project.project_name not in project_parser_files:
                     continue
@@ -1062,7 +1064,7 @@ def track_project_load(self):
 
     # Takes references in 'refs' array of nodes and exposures, finds the target
     # node, and updates 'depends_on.nodes' with the unique id
-    def process_refs(self, current_project: str, dependencies: Optional[Dict[str, Project]]):
+    def process_refs(self, current_project: str, dependencies: Optional[Mapping[str, Project]]):
         for node in self.manifest.nodes.values():
             if node.created_at < self.started_at:
                 continue
diff --git a/core/dbt/parser/read_files.py b/core/dbt/parser/read_files.py
index 3dc2c0fe44c..d07c7fb1d48 100644
--- a/core/dbt/parser/read_files.py
+++ b/core/dbt/parser/read_files.py
@@ -16,9 +16,10 @@
 from dbt.parser.schemas import yaml_from_file, schema_file_keys
 from dbt.exceptions import ParsingError
 from dbt.parser.search import filesystem_search
-from typing import Optional, Dict, List, Mapping
+from typing import Optional, Dict, List, Mapping, MutableMapping
 from dbt.events.types import InputFileDiffError
 from dbt.events.functions import fire_event
+from typing import Protocol
 
 
 @dataclass
@@ -173,12 +174,21 @@ def generate_dbt_ignore_spec(project_root):
     return ignore_spec
 
 
+# Protocol for the ReadFiles... classes
+class ReadFiles(Protocol):
+    files: MutableMapping[str, AnySourceFile]
+    project_parser_files: Dict
+
+    def read_files(self):
+        pass
+
+
 @dataclass
 class ReadFilesFromFileSystem:
     all_projects: Mapping[str, Project]
-    files: Dict[str, AnySourceFile] = field(default_factory=dict)
+    files: MutableMapping[str, AnySourceFile] = field(default_factory=dict)
     # saved_files is only used to compare schema files
-    saved_files: Dict[str, AnySourceFile] = field(default_factory=dict)
+    saved_files: MutableMapping[str, AnySourceFile] = field(default_factory=dict)
     # project_parser_files = {
     #   "my_project": {
     #     "ModelParser": ["my_project://models/my_model.sql"]
@@ -212,10 +222,10 @@ class ReadFilesFromDiff:
     root_project_name: str
     all_projects: Mapping[str, Project]
     file_diff: FileDiff
-    files: Dict[str, AnySourceFile] = field(default_factory=dict)
+    files: MutableMapping[str, AnySourceFile] = field(default_factory=dict)
     # saved_files is used to construct a fresh copy of files, without
     # additional information from parsing
-    saved_files: Dict[str, AnySourceFile] = field(default_factory=dict)
+    saved_files: MutableMapping[str, AnySourceFile] = field(default_factory=dict)
     project_parser_files: Dict = field(default_factory=dict)
     project_file_types: Dict = field(default_factory=dict)
     local_package_dirs: Optional[List[str]] = None