Immediately after topping the Bar exam, Diokno embarked on his law practice at his father's "bupete" or law office, handling and winning high-profile cases including an election case on behalf of his ill father, Sen. Ramón Diokno, who let the young Diokno quickly take over the "bupete". Diokno also successfully fought libel charges against radio personality and Manila Mayor Arsenio Lacson, who was a close friend and would often visit Diokno and his wife in the wee hours at their home in Parañaque to prepare them breakfast. Diokno would in turn edit Mayor Lacson's newspaper columns. Historians learned a few years after Mayor Lacson's sudden passing that Lacson even intended for Diokno to be his running mate, as the Manila Mayor's fame made him the top presidential candidate for the 1965 election. By 1958, Diokno gained enough stature to be selected to join a special committee to investigate the Department of Finance. He was later invited to return to investigate anomalies happening in the Bureau of Supply Corrections.