His early naval career is not recorded, but he served in the 1596 raid on Cádiz under Robert Devereux, Earl of Essex, commanding Vanguard, and was knighted for his part in it.

The Council seem to have got wind of the dispute between Mansell and Heydon, and the Lord Chief Justice wrote to Sir Robert Cecil, urging him to forestall it since the county was "already too much wrought into faction".

[3] In January 1606 he and his nephew, Lewis Mansell, were two of the nobles who performed for the court in 'Hymenaei', a masque written by Ben Jonson for the marriage of Robert Devereux, 3rd Earl of Essex, and Lady Frances Howard.

As treasurer of the navy Robert Mansell was granted funds for "the naval fight to be had upon the river of Thames, for the more magnificent and royal solemnizing of the marriage of the Lady Elizabeth" in February 1613.

[7] In 1613, he was accused of political disaffection and imprisoned in the Marshalsea Prison, but was soon released and seems not to have remained in disfavour for long since he not only retained his offices but in 1615 obtained a monopoly on the manufacture of glass.

He established glass factories in various places, including the first at Newcastle, pioneering the use of sea coal rather than wood in the manufacturing process, and there are records of his later defence of his patents in parliamentary debates.

Contemporary papers suggest this was not in fact intended as a promotion, and that he was deliberately moved to a less influential position because of suspicions of his dishonest administration as treasurer.