State Del. Thomas J. Mooney today named Melvin A. Bilal, a Baltimore lawyer and businessman, to be his running mate in Mooney's long-shot Republican gubernatorial bid.

Bilal, a 43-year-old chief executive of a security firm who became a Republican two years ago, is the second black to announce his candidacy for statewide office in Maryland this year. The other is U.S. Rep. Parren J. Mitchell, a patriarch of the civil rights movement who is running for lieutenant governor with Attorney General Stephen H. Sachs in the Democratic primary.

Though a relative political novice, Bilal could broaden both the political appeal and fund-raising capabilities of Mooney's campaign, which has been overshadowed by the race for the Democratic primary and hampered by an acute shortage of money. Mooney, who has no opposition in the primary, plans to make the centerpiece of his campaign a six-week walk across the state beginning in Oakland on Saturday.

"I've chosen an American success story," Mooney proclaimed at a news conference at the corporate headquarters of Bilal's firm, Howard Security Services Inc. Mooney, a two-term legislator from Takoma Park in Prince George's County, said the Republican team is the only ticket that can "unite the state" by offering candidates from both the Washington and Baltimore areas.

Since announcing his candidacy for governor several months ago, Mooney has encountered a decided lack of enthusiasm among state GOP leaders, some of whom privately complained of his lack of political stature and secretly hoped for a stronger candidate to carry their standard in the fall.

The successful entrepreneur who Mooney added to the ticket is well regarded among party leaders who welcomed him with some fanfare to the GOP in 1984 and who recommended that Mooney seek him to run as his lieutenant governor candidate.

"Mel Bilal is the best choice Mooney could have made," said state Republican chairman Allan C. Levey. For Republicans, Bilal is a symbol of the free enterprise system; as a successful black, he may attract black and liberal Democrats.

Levey credited Bilal with building bridges to the state's black community in behalf of the Reagan-Bush presidential campaign two years ago.

Bilal is a former world class sprinter. He is a graduate of the University of Illinois and the University of Pennsylvania Law School. He is an Army veteran and a former teacher. Bilal founded Howard Security Services, a company he began 13 years ago with a $100 investment and now employs 800 people and provides security for clients such as the Rouse Corp. and federal agencies.

Bilal, whose office is adorned with a Reagan-Bush "Bringing America Back" campaign poster, has been wooed by national Republican figures since he left the Democratic Party two years ago. He has dined with George Bush, joined the vice president's political action committee, and in August will participate in the White House small business conference.

Bilal's conversion to the GOP came just one year after he served as a finance chairman for the Baltimore mayoral campaign of William H. Murphy Jr., a liberal Democrat. Bilal said today that that role was based more on emotion than philosophy and reflected his view that "it was time to have a black mayor."

Bilal sang the praises of Reagan and chastised the Democratic Party for promoting "well-intentioned . . . hand-out programs" that make blacks "dependent" on government