Washington Gas Light Co, yesterday reported first-quarter net income of $18.859 million ($4.13 a share), down from $19.448 million (4.27) in the same quarter last year. Company officials said the decline was caused in part by warmer temperatures during the first quarter this year than during the same period in 1980.

Operating revenues were $219.278 million compared with $185.527 million for the same three months last year.

Net income for the 12 months ended March 31 was $17.497 million compared with $17.593 million for the previous 12 months. Revenues for the year ended March 31 werre $497.746 million compared with $409.732 million a year before.

John Hanson Savings & Loan Inc. of Forestville, Md., reported net earnings for the nine months ending March 31 of $437.740 (37 cents a share) compared with $97.910 (8 cents) during the same nine months of its 1980 fiscal year. The results represented and increase of more than three times the previous year's profits.

According to John Hanson Chairman Charles A. Dukes Jr., assets for the 12-month period ended March 31 increased by $43.085 million, or 30.1 percent.Total assets as of March 31 were $186.423 million. Deposits were $161.071 million.

Dukes attributed the increases to aggressive management practices, expansion of services and general overall growth. The S&L, which has assets of approximately $188 million, operates 10 branches in suburban Maryland.

The Washington Corporation, a real estate company with headquarters in Chevy Chase, reported first-quarter net income of 1981 of $150,007 (6 cents a share) compared with $372,423 (16 cents) for the same quarter in 1980. The latest results also included an operating loss of $20,591 after the effect of income taxes and $170,598 in extraordinary gains derived from forgiveness of bank indebtedness.