Is Regina King related to Mabel King?

Also asked, why did Mabel King leave what's happening?

Due to disagreements with the direction the creators wanted to take the series, King left What's Happening!! in 1978 after two seasons. That same year, she reprised the role of Evillene for the 1978 film version of The Wiz.

Also Know, how old was Mabel King when she died? 66 years (1932–1999)

Beside above, what did Mabel King died from?

Diabetes

Who died on what's happening?

Fred Berry, the portly actor best known for his comic turn as "Rerun" on the hit ABC sitcom What's Happening!! in the late 1970s, died in 2003 at the age of 52 in Los Angeles.