diff --git a/lib/configparser/Cargo.toml b/lib/configparser/Cargo.toml
index 0245c40556..a2edac7b04 100644
--- a/lib/configparser/Cargo.toml
+++ b/lib/configparser/Cargo.toml
@@ -14,6 +14,7 @@ pest_derive = "2.1.0"
 shellexpand = "1.0.0"
 
 [dev-dependencies]
+lazy_static = "1.3.0"
 minibench = { path = "../minibench" }
 tempdir = "0.3.7"
 
diff --git a/lib/configparser/src/hg.rs b/lib/configparser/src/hg.rs
index 4928b1bf62..e7d2e0068e 100644
--- a/lib/configparser/src/hg.rs
+++ b/lib/configparser/src/hg.rs
@@ -514,8 +514,18 @@ mod tests {
 
     use crate::config::tests::write_file;
 
+    use lazy_static::lazy_static;
+    use std::sync::Mutex;
+
+    lazy_static! {
+        /// Lock for the environment.  This should be acquired by tests that rely on particular
+        /// environment variable values that might be overwritten by other tests.
+        static ref ENV_LOCK: Mutex<()> = Mutex::new(());
+    }
+
     #[test]
     fn test_basic_hgplain() {
+        let _guard = ENV_LOCK.lock().unwrap();
         env::set_var(HGPLAIN, "1");
         env::remove_var(HGPLAINEXCEPT);
 
@@ -540,6 +550,7 @@ mod tests {
 
     #[test]
     fn test_hgplainexcept() {
+        let _guard = ENV_LOCK.lock().unwrap();
         env::remove_var(HGPLAIN);
         env::set_var(HGPLAINEXCEPT, "alias,revsetalias");