NEW YORK (AP) _ The CBS Inc. board decided unanimously Wednesday to elect company founder William S. Paley as chairman and Laurence A. Tisch as president and chief executive, jobs they have held on an interim basis since September.

The selections, which had been widely expected and were applauded by analysts, follow a tumultuous period for the broadcasting-entertainment giant.

It fought off an unwelcome takeover bid in 1985 and has been forced to cut costs in the face of persistent sluggishness in demand for network advertising.

In September 1986, Thomas H. Wyman resigned as chairman, president and chief executive after reportedly telling the board that he had approached Coca-Cola Co. about making a friendly takeover offer for CBS.

The board promptly turned to board members Paley and Tisch to guide the company in the transition. It named two committees to search for a permanent choices, and Wednesday's announcement said that ''with this action ... (they) have completed their functions.''

Paley, 85, founded CBS in 1927 as a radio network but was forced to retire as chairman in 1983 to make room for Wyman.

Tisch, 63, is chairman of Loew's Corp., CBS's biggest shareholder. Wyman had invited him to join the CBS board in October 1985, but Tisch reportedly grew increasingly critical of how the company was managed.

In a statement accompanying the CBS announcement, Tisch noted that he had initially been asked and expected to serve ''only for for a brief transition period.''

But he said he had ''become increasingly impressed over the last four months with the strengths and potential of this great company and of its people.''

''I look forward to continuing to work with Bill Paley and the board to build upon the traditions and spirit which have guided CBS over the last 60 years,'' he said.

Paley said the board has been impressed by Tisch's ''commitment to CBS and its values and by his grasp of the strategic and operating issues confronting our company'' in the months they have worked together.

Paley said he hopes to ''continue working closely with CBS programming staffs on the creativity and quality of our offerings.''

Peter Appert, a media analyst for the investment firm C.J. Lawrence & Co., said the board's latest decision came as no surprise.

John Reidy, media analyst for the investment firm Drexel Burnham Lambert Inc., agreed, adding ''They have been working well in tandem.''

''It's healthy for the company to have both the administrative skills of Laurence Tisch and the longterm broadcasting perspective of Paley both in place,'' he said.

On trading on the New York Stock Exchange, the price of CBS stock rose $1.75 to $138.75 a share.

Tisch is seen as having the expertise to keep costs under control. Over the past year, CBS has laid off between 1,500 and 2,000 employees, sold its television station in St. Louis and closed a technology center in Connecticut.

In October, CBS sold its book publishing division for $500 million to Harcourt Brace Jovanovich Inc., a Florida textbook publisher. It hired a management conulting firm to review how the company operates.

A new organizational structure was imposed at the CBS Broadcast Group in December which it said would ''provide us with a tighter management core designed to repond to a faster-moving business environment.''

Drexel's Reidy said an unresolved question is whether CBS will find it necessary to sell more assets. The board decided in November against selling its records division.

For the first nine months of 1986, CBS reported a profit of $152.1 million on revenue of $3.57 billion, with 57 percent of revenue from its broadcast operations, 29 percent from records and 14 percent from publishing, including both its magazines and the since-sold book division.

Reidy said asset sales to date should significantly reduce the large debt that CBS incurred in fending off a hostile takeover bid in 1985 by Atlanta broadcaster Ted Turner. Lawrence's Appert said the company ''is no longer in a shrinking mode.''

They both said CBS may want to buy another television station if the opportunity arises. They said the four stations CBS owns reach about 19 percent of the nation's television households, while ABC's stations give it a reach near the permissible limit of 25 percent and NBC's stations reach about 21 percent.