He started out as a police detective in Laric, where he investigated a cat burglar who intimidated his victims by impersonating a monitor. He nearly caught the perp, but was beaten to the punch by an actual monitor, who, offended by the presumpition of a mere human in impersonating one of the elite warrior-monks, killed the burglar and left his destroyed disguise on the ground next to him as a message.
MacNeil went on to become a henchman-for-hire to various adventurers, during which time his life was once saved by a monitor. Monitors are something of a recurring theme in MacNeil's life.
At some later point, MacNeil met an unnamed woman and had a similarly unnamed son. He had in fact been heading to the Chasm to pass on his sword and armour to said son while he's still well enough to make the trip. Sadly, he apparently wasn't well enough to make the trip, as he died during the night. The party delivered the material goods he'd had on his person to the son and were given a chest of potions and martial scrolls for their trouble.