William McMahon McKaig (July 29, 1845 – June 6, 1907) was an American politician.

Born in Cumberland, Maryland, McKaig attended the Carroll School and the Allegany County Academy.

He was appointed city attorney of Cumberland in 1876, and was elected to the Maryland House of Delegates in 1877.

He was also elected to the Maryland State Senate in 1887, and as mayor of Cumberland in 1890.

He died in Cumberland, and is interred in Rose Hill Cemetery.