Marriott Corp., United Financial Banking Cos. and MIW Investors of Washington, yesterday reported substantially better earnings for 1984.

Marriott, the Bethesda food and lodging giant, announced profits of $41.8 million in the fourth quarter ($1.57 a share), up 35 percent from $31 million ($1.11) in the same period last year. Sales jumped by 19 percent to $1.1 billion from $941.8 million.

For the year as a whole, profits totaled $139.8 million ($5.18), up 21 percent from $115.2 million ($4.15) a year ago. Revenue climbed 19 percent to $3.5 billion from $2.95 billion in 1983.

President J. W. Marriott Jr. said sales and profits have doubled since 1980 and increased tenfold since 1971. He attributed 1984's results to the strong economy, increased travel and reduced interest rates. The company sold its Illinois theme park last year and is now in the process of selling another in California. Income from these operations declined to $4.5 million in 1984 from $6.8 million the previous year.

* United Financial of Vienna, parent company of the Business Bank, which makes loans to small businesses and professionals, experienced very rapid growth last year. Net income rose 135 percent to $713,165 from $302,872, but earnings per share dropped slightly to $1.22 from $1.25 as the number of shares outstanding increased sharply.

Fourth-quarterearnings totaled $193,815 (35 cents), up 111 percent from profits of $91,920 (38 cents) in the comparable 1983 period. Total assets doubled to $49.7 million, and liabilities increased 129 percent to $43 million by year's end.

* MIW Investors of Washington, a real estate investment and development company, posted a record gain of 74 percent in 1984. Profits jumped to $3.7 million (81 cents a share), compared with $2.1 million (56 cents) in 1980.

Fourth-quarter income was affected by the acquisition in September of First American Savings and Loan Association. Earnings were $2.7 million (44 cents), a jump of 260 percent from the $753,000 (18 cents) registered in the same period the year before.