Henry Christensen III, a renowned trust and estates lawyer whose conflicted loyalties cast him as a pivotal witness in the trial of Anthony D. Marshall for defrauding his mother, Brooke Astor, the philanthropist and doyenne of New York society, died on Nov. 3 2017 in Brooklyn. He was 72. Mr. Christensen was esteemed but largely unknown outside his field until 2006, when Mr. Marshall, a Broadway producer and former ambassador who was Mrs. Astor’s only son, was accused of swindling millions of dollars from his mother after she was found to have dementia. As her lawyer since 1991, Mr. Christensen was criticized — but never charged criminally — after overseeing amendments to Mrs. Astor’s will in 2003 and other transactions that profited her son and daughter-in-law. Mr. Christensen was born on Nov. 8, 1944, in Jersey City to Henry Christensen Jr., a numismatist, and the former Margaret Louise Brooke. He was known as Terry, from the Latin word tertium, meaning third. Raised in Madison, N.J., he earned a bachelor’s degree in literature from Yale in 1966 and graduated from Harvard Law School. He married Constance Crumpton, his high school sweetheart, in 1967. In addition to her, Mr. Christensen, who lived in Brooklyn, is survived by his children, Alexander, Gus, Elizabeth and Katherine Christensen; four grandchildren; and a sister, Karen Cheeseman. In 1969, Mr. Christensen joined the Manhattan law firm Sullivan & Cromwell, which had represented Mrs. Astor since her husband, Vincent, died in 1959. He was a partner there from 1977 until he left to join another firm, McDermott Will & Emery, in 2007.