Born in San Juan, Puerto Rico, Casellas graduated from the Academia del Perpetuo Socorro in 1953, with High Honors.
Served in the United States Army as a Second Lieutenant in the United States Army Armor Branch from 1961 to 1963, and remained in the United States Army Reserve in the Judge Advocate General's Corps from 1963 to 1967.
He served as Secretary of the Treasury for the Commonwealth of Puerto Rico from 1973 to 1976, thereafter returning to private practice in San Juan from 1977 to 1994.
On June 21, 1994, Casellas was nominated by President Bill Clinton to a seat on the United States District Court for the District of Puerto Rico vacated by Jaime Pieras, Jr. Casellas was confirmed by the United States Senate on September 28, 1994, and received his commission on September 29, 1994.
Judge Casellas died on November 22, 2017, at Auxilio Mutuo Hospital in Hato Rey, Puerto Rico.