Jordin Canada, who made nine of 10 from the free-throw line, had 15 points for Los Angeles (10-18).
Sparks coach Curt Miller became the fifth-fastest in WNBA history to reach 150 career wins and became one of three coaches (Van Chancellor and Lin Dunn) with at least 275 collegiate and 150 professional wins.
Brittney Sykes hit a jumper to give Washington an eight-point lead with 6 minutes to play, and she made one of two free throws to make it 78-75 with 3 minutes remaining. Karlie Samuelson answered with two foul shots, and Ogwumike followed with a short jumper that gave the Sparks the lead for good with 2:14 left and Samuelson added a three-pointer about a minute later to make it 83-78.
L.A.’s Layshia Clarendon was fouled by Sykes on a drive to the basket with about 33 seconds to play and a scuffle ensued. After a video replay, Sykes and Clarendon were both ejected from the game and Rae Burrell made the ensuing two free throws to give the Sparks an 85-80 lead.
Tianna Hawkins scored 23 points on nine-of-13 shooting to lead Washington (13-14). Sykes added 17 points and six assists.
The Sparks have lost 11 of their last 14 games.
The Mystics, who beat Los Angeles 79-77 on Friday night, had won back-to-back games for the first time since winning three straight from June 16-22.
Regular starters Elena Delle Donne (ankle), Ariel Atkins (ankle) and Shakira Austin (hip) missed their eighth consecutive game for the Mystics. Delle Donne has missed nine straight and Austin has been out since June 25.