Mantell was born in Scotland and first appeared on stage in Belfast, Northern Ireland; for a time he used the stage name Robert Hudson, but he reassumed his name Mantell after he joined the company of Helena Modjeska in 1878 and came to the United States. His first professional appearance was at the Theatre Royal in Rockdale in 1876. His first role with Dame Modjeska was as Tybalt in Hamlet at the Leyland Opera House in Albany, New York.
He returned to the UK, but met with little success and went back to the US permanently, where for years he toured with his own company. He was not particularly popular in New York City, so he toured other cities in the country. He was an indefatigable worker—something of a war horse—and he was constantly on the road with this troupe. One way for him to keep a leading actress with this kind of workload was to marry her, which he did four times. He married his last wife, actress Genevieve Hamper, thirty-four years his junior, until his death.
Over the years, Mantell played in Hamlet, Othello (as both Iago and Othello, alongside Hamper’s Desdemona), Romeo & Juliet (alongside Hamper), Richard III, King Lear (a role he especially enjoyed), Macbeth, The Merchant of Venice, Julius Caesar, and King John. Despite his failure to excite the critics and public in London and New York, he was a well-respected Shakespearean actor on the road. He also played in eight films between 1896 and 1923. Clarence J. Bulliet wrote his biography, Robert B. Mantell’s Romance, in 1918.