In January 1870 Robertson was diagnosed as suffering from heart disease. He continued to write, and 1870 saw the production of "Progress" (adapted from Sardou) at the Globe, "The Nightingale", a drama, at the Adelphi Theatre and his final work for the Prince of Wales's – "M. P.". Robertson was an early beneficiary of improved financial terms for playwrights; the practice of payment by royalties was not widespread until the 1880s, but the management of the Prince of Wales's had paid him £1 a night for "Society" in 1865, and by the time of this final piece his nightly fee had risen to £5. He was also – most unusually for the period – paid for revivals. He was unable to supervise the production of "M. P." or even to attend the first night. The company went to his house and gave him a private performance.