Answer
No, Sensenbrenner was not the senior-most member in the House at his retirement. He was the second most senior. The most senior member in the House at the time of Sensenbrenner's retirement was John Dingell. Dingell served in the House from 1955 to 2015, and was the longest-serving member in House history. Sensenbrenner served in the House from 1979 to 2019, and was the eighth-longest-serving member in House history.