Pope John Paul II yesterday named Bishop Roger M. Mahony to succeed Cardinal Timothy J. Manning as archbishop of Los Angeles, the nation's largest Roman Catholic archdiocese, with 2.5 million Catholics.

Mahony, a native of Los Angeles, has been involved for two decades in the battle over conditions for farm workers in California. In 1975 he took a brief leave from church duties to head the state's Agriculture Labor Relations Board.

Nearly two years before the National Conference of Catholic Bishops issued its pastoral condemning nuclear war, Mahony issued his own pastoral letter to the 127,000 Catholics in the Stockton diocese, to which he was named bishop in 1980.

In a 1982 pastoral, he urged sensitivity to the culture of Hispanics, who make up roughly 45 percent of the Stockton diocese. In April of this year, the diocese established a cooperative to help family farmers purchase supplies.

In light of Mahony's activism, some observers were stunned in February when he issued a directive requiring approval from his office before any outside speakers on Catholic teaching could be brought into the diocese.

Manning, who last November reached the church's mandatory retirement age of 75, will continue as a member of the College of Cardinals. Mahony, 49, will be promoted to the rank of archbishop.