Representative Nita M. Lowey, the first woman to lead the powerful House Appropriations Committee, unexpectedly announced on Thursday that she would not seek re-election in 2020.

Ms. Lowey, a New York Democrat who was first elected in 1988, is a staunch ally of House Speaker Nancy Pelosi, and she played a key role in the budget negotiations that helped end the country’s longest government shutdown earlier this year. She represents the northern suburbs of New York City, including Westchester and Rockland Counties.

When Ms. Lowey, 82, arrived in Congress after defeating a two-term Republican incumbent, women made up less than 7 percent of the House of Representatives. She was consistently regarded as a pioneer, from her stint as the first woman to lead the campaign arm for House Democrats, to her ascent this year to chairwoman of the Appropriations Committee.

But she was also among a group of powerful, long-serving New York House members who had drawn energetic primary challengers for next year. Mondaire Jones, a 32-year-old Harvard Law graduate who had worked in the Justice Department under former President Barack Obama, raised more than $218,000 last quarter for his primary challenge to Ms. Lowey, arguing that the district needed a more energetic representative.