diff --git a/contrib/automation/hgautomation/aws.py b/contrib/automation/hgautomation/aws.py
--- a/contrib/automation/hgautomation/aws.py
+++ b/contrib/automation/hgautomation/aws.py
@@ -54,6 +54,7 @@
 
 AMAZON_ACCOUNT_ID = '801119661308'
 DEBIAN_ACCOUNT_ID = '379101102735'
+DEBIAN_ACCOUNT_ID_2 = '136693071363'
 UBUNTU_ACCOUNT_ID = '099720109477'
 
 
@@ -807,7 +808,7 @@
     return image
 
 
-def ensure_linux_dev_ami(c: AWSConnection, distro='debian9', prefix='hg-'):
+def ensure_linux_dev_ami(c: AWSConnection, distro='debian10', prefix='hg-'):
     """Ensures a Linux development AMI is available and up-to-date.
 
     Returns an ``ec2.Image`` of either an existing AMI or a newly-built one.
@@ -824,6 +825,13 @@
             'debian-stretch-hvm-x86_64-gp2-2019-09-08-17994',
         )
         ssh_username = 'admin'
+    elif distro == 'debian10':
+        image = find_image(
+            ec2resource,
+            DEBIAN_ACCOUNT_ID_2,
+            'debian-10-amd64-20190909-10',
+        )
+        ssh_username = 'admin'
     elif distro == 'ubuntu18.04':
         image = find_image(
             ec2resource,
diff --git a/contrib/automation/hgautomation/cli.py b/contrib/automation/hgautomation/cli.py
--- a/contrib/automation/hgautomation/cli.py
+++ b/contrib/automation/hgautomation/cli.py
@@ -352,7 +352,7 @@
         '--distro',
         help='Linux distribution to run tests on',
         choices=linux.DISTROS,
-        default='debian9',
+        default='debian10',
     )
     sp.add_argument(
         '--filesystem',
diff --git a/contrib/automation/hgautomation/linux.py b/contrib/automation/hgautomation/linux.py
--- a/contrib/automation/hgautomation/linux.py
+++ b/contrib/automation/hgautomation/linux.py
@@ -21,6 +21,7 @@
 # Linux distributions that are supported.
 DISTROS = {
     'debian9',
+    'debian10',
     'ubuntu18.04',
     'ubuntu19.04',
 }
@@ -190,14 +191,18 @@
 cat << EOF | sudo tee -a /etc/apt/sources.list
 # Need backports for clang-format-6.0
 deb http://deb.debian.org/debian stretch-backports main
+EOF
+fi
 
+if [ "$LSB_RELEASE" = "stretch" -o "$LSB_RELEASE" = "buster" ]; then
+cat << EOF | sudo tee -a /etc/apt/sources.list
 # Sources are useful if we want to compile things locally.
-deb-src http://deb.debian.org/debian stretch main
-deb-src http://security.debian.org/debian-security stretch/updates main
-deb-src http://deb.debian.org/debian stretch-updates main
-deb-src http://deb.debian.org/debian stretch-backports main
+deb-src http://deb.debian.org/debian $LSB_RELEASE main
+deb-src http://security.debian.org/debian-security $LSB_RELEASE/updates main
+deb-src http://deb.debian.org/debian $LSB_RELEASE-updates main
+deb-src http://deb.debian.org/debian $LSB_RELEASE-backports main
 
-deb [arch=amd64] https://download.docker.com/linux/debian stretch stable
+deb [arch=amd64] https://download.docker.com/linux/debian $LSB_RELEASE stable
 EOF
 
 elif [ "$DISTRO" = "Ubuntu" ]; then
@@ -279,8 +284,8 @@
     PACKAGES="$PACKAGES linux-tools-common"
 fi
 
-# Ubuntu 19.04 removes monotone.
-if [ "$LSB_RELEASE" != "disco" ]; then
+# Monotone only available in older releases.
+if [ "$LSB_RELEASE" = "stretch" -o "$LSB_RELEASE" = "xenial" ]; then
     PACKAGES="$PACKAGES monotone"
 fi