"Mark" is Marcus Aurelius Antoninus (121-180 AD), who reigned as Emperor from 161 to 180 AD. Hadrian, who had no natural children, adopted Antoninus Pius as his son and heir, on condition that he in turn adopted Marcus Aurelius. The Florentine political thinker, Nicolo Macchiavelli ("Discourses on the First Decade of Titus Livy," Book 1, Chapter 10) considered him to be one of the "five good emperors" (the others being his immediate predecessors, Nerva, Trajan, Hadrian and Antoninus Pius, all of whom owed their position to adoption). He was succeeded by his natural son, Commodus, widely considered to have been among the worst emperors. The historian Edward Gibbon went further in describing the period covered by the reigns of these emperors as "...the period in the history of the world during which the conditions of the human race was most happy and prosperous." He was a noted philosopher, as well as a statesman and general, his "Meditations" being one of the key texts of Roman Stoicism.
For a modern biography see A.R. Birley, Marcus Aurelius: A Biography.