Comcast said first-quarter net income fell 12.5% as revenue from its bedrock cable business dipped and ad revenue from its NBCUniversal revenue networks fell.

The Philadelphia owner of NBC, Peacock and the Universal movie studio said overall revenue fell slightly to $29.89 billion, compared with $30.06 billion in the year-earlier period. Net income came to 89 cents per share. Earnings, adjusted for costs tied to amortization and investment, came $1.09 per share.

In a statement, Comcast CEO Brian Roberts said the company was showing financial discipline during a challenging environment for the media industry. “With our significant free cash flow generation, disciplined approach to capital...