Finstad received the Frank Wardlaw Prize for literary excellence for her first book, Heir Not Apparent (1984), drawn from her experiences as a young law clerk and trial attorney investigating claims to the billion-dollar estate of industrialist, aviator and filmmaker Howard Hughes, who appeared to have died without a valid will.

Her second book, Ulterior Motives: The Killing and Dark Legacy of Tycoon Henry Kyle (1987), took her into the genre of the non-fiction true crime novel created by Truman Capote’s In Cold Blood, centering on a spectacular murder trial which revealed that self-made millionaire Kyle, shot by his older son, had a violent Jekyll-Hyde personality and a lifetime of secrets, including a hidden wife and daughter. According to Publishers Weekly, much of the book "is tedious in its over-attention to detail."

Finstad wrote the national bestseller Sleeping with the Devil (1991), the true story about the murder-for-hire of Barbra Piotrowski, a Californian beauty queen in a destructive love triangle with a married Texas health club tycoon named Richard Minns, alleged to have hired the assassins who shot and paralyzed Piotrowski. Finstad’s research led to Minns’s arrest for felony passport fraud after he fled the country using assumed names.

In 1997, Finstad wrote a biography of Priscilla Presley (1997) called The Untold Story of Priscilla Beaulieu Presley (1997). Finstad offered a detailed account of the courtship and marriage of Elvis Presley and 14-year-old Priscilla Beaulieu based on extensive interviews with family, close friends, classmates, co-stars and Priscilla Presley. The latter filed a lawsuit against Currie Grant, a former Army buddy of Elvis's, for his claim that he had sex with her in exchange for introducing her to the singer. Finstad had endeavored to settle the disagreement between Grant and Priscilla at her house, but the result was that Priscilla stormed out of the house. Finally, a judgment was entered against Grant, whom Priscilla had sued for at least $10 million. He was ordered to pay $75000. However, neither Finstad nor her publisher was a party to the lawsuit, and Finstad and her publisher say they stand by their account of the affair in the book.

Finstad also wrote biographies of Natalie Wood (2001) and Warren Beatty (2005).

Sleeping With the Devil and The Biography of Natalie Wood were adapted for television — the former fairly successfully but the latter, directed by Peter Bogdanovich, less so.

Natasha: The Biography of Natalie Wood was named the best film book of 2001 by the San Francisco Chronicle. Finstad’s most recent biography, Warren Beatty: A Private Man, was selected as one of the top five entertainment books of 2005 by the Times of London.