The Spanish striker was well-placed at the far post for his volley after a corner kick was unintentionally flicked on by a Mallorca defender in the 86th minute.
Betis had hit the woodwork three times in the first half and had various chances after the break at Benito Villamarin stadium in Seville.
“From the opening whistle we gave it a tremendous effort,” Betis coach Pepe Mel said. “We have to be satisfied with the work both in defense and attack of all 11 players.”
It was Castro’s second late winner in as many weeks as Betis maintained its perfect start to the season, lifting the promoted club to the top of the league standings along with Real Madrid and Valencia. Defending champion Barcelona is two points behind.
Elsewhere, Levante failed to take advantage of its extra man in the final 10 minutes after Racing Santander midfielder Papakouly Diop was sent off with two yellow cards and had to settle for a 0-0 away draw.
Later, Athletic Bilbao visits Espanyol as both teams look for their first win. Osasuna hosts Sporting Gijon, and Zaragoza looks to bounce back from its opening round 6-0 thrashing by Madrid at the promoted Rayo Vallecano.
On Saturday, Karim Benzema scored a double as Madrid beat Getafe 4-2, while Barcelona squandered an early lead with a pair of defensive lapses that let Real Sociedad rally to a 2-2 draw.
Also, Roberto Soldado gave Valencia a 1-0 win over Atletico Madrid with his fourth goal of the season, and a 10-man Villarreal salvaged a 2-2 draw with Sevilla.
