Nneka Ogwumike tied a season high with 24 points, had nine rebounds and blocked two shots in the final 10 seconds to lead Los Angeles (12-5), which extended its winning streak to eight games over the Storm. Seattle last beat the Sparks in 2011.
"We had several options," Beard said about her winning shot. "When Lindsey (Harding) drove, it opened up the lane for me to pull up for a jump shot. It just came after the first option wasn't there, the second option wasn't there and I became the third or fourth option."
Harding had 10 points and eight assists for the Sparks. Candace Parker, who entered the game averaging 18.4 points, had just seven points.
Camille Little scored 22 points to lead Seattle (6-10), which has lost four of five and seven of nine. Shekinna Stricklen added 13 points and Tina Thompson had nine.
Parker hit one of two free throws to cut the lead to 62-61 with a minute left. Temeka Johnson responded with a drive to make it 64-61 for Seattle before Harding made a layup to make it 64-63.
Stricklen missed a 3 with 23 seconds left that the Sparks rebounded to set up Beard's winning shot.
Seattle had a chance to regain the lead, but Ogwumike blocked a Johnson shot on a drive to the basket with 1 second left. The ball went out of bounds to Seattle, which had 0.6 seconds for a final play. Thompson got the inbound pass but Ogwumike blocked the shot.
"Coach (Carol Ross) said we just have to get one stop and we weren't going to try to foul and if we did it was going to be on the dribble," Ogwumike said. "I think we did a good job of getting that stop.
"We switched off and at the end we even had to get another step with 0.6 on the clock. I think we did a really good job of understanding what was on the line because we were only up by one."
Seattle coach Brian Agler watched his team come up short on the final shots.
"We were trying to get Camille on a slip play and I could tell she was open but I couldn't tell how Tanisha (Wright) was guarded out of bounds," Agler said. "They may have jumped up and took that pass away. It was a broken play. Temeka got the ball and got it to the rim and it went out of bounds. With 0.6 seconds we got what we didn't want and it (the shot) didn't go down."
Little scored a basket midway through the fourth quarter to give Seattle a 60-54 lead, but Ogwumike answered with two baskets to keep the Sparks close. Ogwumike hit 10 of 14 shots from the field.
Ogwumike scored eight and Kristi Toliver five during a 17-4 run, including a 10-0 streak, as the Sparks rallied from a 12-point deficit in the third quarter to lead 50-49. Little had eight points and hit four straight free throws to give the Storm a 53-52 lead after three quarters.
"I was very proud of our second-half effort because it was a grind," Ross said. "We hadn't shown a whole lot of resilience in the past in fighting our way out of tough situations so I was real pleased with our fight."
Little and Stricklen scored 10 each to help Seattle to a 39-31 halftime lead. Stricklen had eight in the first quarter and Little tallied eight in the second quarter when the Storm led by 11.
The Sparks had a six-game winning streak snapped with a 90-84 loss to Phoenix on Thursday.