Walt Masterson was born on June 22, 1920 in Philadelphia. He attended Catholic schools, first Holy Innocents and then North East Catholic High School. He played some semipro ball, for the Landreth Seed Co. team of Bristol, Pennsylvania in 1938.

He was spotted by a Washington scout who signed him. He spent a couple of months traveling with the Nats in 1938 as a non-playing rookie. A few weeks before spring training was over in 1939, he was one of nine sent to the minors for seasoning at Charlotte in the Class-B Piedmont League. He was recalled to Washington in April, and debuted in May, when he was still 18 years old. By seasonís end, he had two wins and two losses.

In 1940, Washington was ready to send him to the minors, but the young pitcher overcame what some termed timidity and came through with three impressive innings late in March. During spring training, he had trouble seeing and wore eyeglasses from that point forward. He won his first game in 1940 with four innings of one-hit relief. His ERA for the year was 4.90, higher than the teamís overall 4.59, and his record was a depressing 3-13. 

It was on-the-job training for Walt, and he didnít get any better. He had six starts and 28 relief appearances, and pitched to a 5.97 ERA in 1941. In 1942, he turned it around, cutting his ERA all the way down to 3.34 and threw four shutouts before the year was done.

He then joined the Navy. He had tried to enlist before, but been rejected due to color blindness. In early October, he was accepted on his second attempt to enlist and served as a chief specialist in the United States Navy Reserve for three years, from October 1942 to September 1945. Heíd played a great deal of baseball in the Navy and in July 1943, while stationed at the submarine base at Pearl Harbor, he threw a no-hitter. With the Navy, he was 17-1 in 1943, 20-4 in 1944, and 18-0 in 1945. But his military service was not all fun or games. He served in naval combat at the U. S. base on Midway Island and during the battle to recapture Guam in 1944.

After his separation from the service, honorably discharged as a chief petty officer, in the submarine section at New London, Connecticut, he was able to get into four September games for the Senators. Over his four appearances in 1945, he won one game but lost two despite an overall 1.08 ERA in 25 innings.

He worked in the off-season as a salesman of electrical goods for Porter Engineering and he co-owned the dry cleaning firm of Masterson and Evans.

In 1947, he came through with perhaps, the best season of his career. He threw 34 consecutive scoreless innings before the streak was snapped and he was named to the 1947 American League All-Star team. He matched his career high with four shutouts.

He complained of shoulder stiffness near the start of spring training 1948 and didnít work until pitching three innings of an intrasquad game in late March. He pitched well enough to make the 1948 All-Star team again, but worked about 50 fewer innings to a 3.83 earned run average with an 8-15 record. 

In 1949, Walt changed teams, but not before having an appendectomy in April and was traded to the Boston Red Sox. In 1950, the Red Sox were an offensive juggernaut, having scored more runs than any other team in franchise history, and in a 154-game season. In his up-and-down earned run averages, Walt was up again, to a 5.64 ERA (with a record of 8-6). In both 1949 and 1950, he split his time between starting and relieving (starting 15 of 33 games in 1950) and continued to struggle with control, walking more than he struck out. 

The 1951 season saw Walt work almost exclusively in relief. He appeared in 30 games, closing 10 of them, but only had one start. He cut his ERA by more than two runs to 3.34. Masterson rejoined the Senators in 1952. Heíd appeared in five games for the Red Sox and given up 12 earned runs in 9 1/3 innings. He started 21 games over the rest of the season for the Senators, and acquitted himself well with a 3.70 ERA.

Walt announced his retirement in January 1954, saying he wanted to devote full time to his work as a sales rep for a shoe firm in Freeport, Maine. He was being paid $18,000 a year to sell shoes, and the Senators team was only offering $4,000. He was out of organized baseball in 1954 and 1955, but was still on a Senators contract. He was given his release in April 1956, and a week and a half later, he signed as a free agent with the Detroit Tigers, who released him at the end of the season.

His final season of pro ball was in the Pacific Coast League in 1957, working for the San Francisco Seals. He only worked in six games and by July he was back home in Maryland. He took up work in the flour brokerage business, living in Washington while working for a group of millers in Kansas. The Masterson family lived in Woodville, Virginia, where Walt served on the school board.

In November 1979, he was the head baseball coach at George Mason University, where he served for two years, 1980 and 1981.

Walt Masterson moved to North Carolina around 1985 and passed away from a stroke on April 5, 2008, at Duke University Hospital in Durham, North Carolina. He was 87 years old.