In the more recent case, involving a 25-year-old model, a district attorney said there was insufficient evidence to prove guilt beyond a reasonable doubt.
For the older case, prosecutors said the statute of limitations had expired.
Mr Cosby, 78, was charged in a sex assault case last week in Pennsylvania. He has denied the claims.
Dozens of women have come out with allegations against him including drugging, sexual harassment, sexual assault and rape, dating back decades.
In one of the LA cases, model Chloe Goins claimed Mr Cosby drugged and molested her at the Playboy Mansion in 2008 when she was 17.
Mr Cosby's lawyers have claimed he was not at the same party as Ms Goins.
In the 1965 case, the alleged victim claims Mr Cosby gave her an alcoholic drink at a jazz club in Hollywood, took her to a house and forced her to have sex with him.
At the height of his fame from The Cosby Show, the comedian was the highest paid entertainer on US television.
On Wednesday, his wife learnt that she would not have to give evidence in a defamation case filed by seven women accusing the comedian of sexual assault.
Prosecutors in Los Angeles have declined to charge comedian Bill Cosby for two alleged sex assault cases, one from 1965 and one from 2008.