After the Master's defeat, Marcus wandered aimlessly, looking for fellow super mutants. 2185年，马可世在荒漠中遇到了钢铁兄弟会的圣骑士雅各布, who had pledged himself to the extermination of all mutants. 。在持续好几天的战斗后，双方发现很难推倒对方，所以干脆放弃，Eventually, they realized the pointlessness of their fight, and broke down in joined laughter, beginning a strong friendship between them. 他們结伴踏上了新的旅途。尽管一路上他们都在为教主的野望和兄弟会的教义and Brotherhood doctrine and whether or not the Master could truly neurolink his biology into the Cathedral computer network.争吵，他们的友情还是很可靠的。一个超级变种人加一个身穿动力盔甲的战士，这对强大的組合吸引了很多追随者寻求他们的庇护，有人类，有僵尸，也有超级变种人。In Fall of the same year, 他們建立了破碎丘，作为一个各种族共同生活的社区。
终于，他带领一群渴望平静生活的超级变种人来到了莫哈维废土，并在黑山建立了一个和平的避难所。This fell apart when另一群智力比较低下的变种人，由夜行族塔比瑟的带领下赶走马可世一行人。到了2281年，马可世已經在偏远的查尔斯顿山脉旁边建立了另一个“世外桃源”。为了纪念他的老友，他把这里命名为雅各布镇。一些精神分裂的夜行族也在这里接受他的庇护，他希望收留的前英克雷科学家亨利医生能够治好那些夜行族的精神疾病。Marcus will also greet the Courier and warn him about the nightkin within Jacobstown when the Courier enters Jacobstown for the first time.
Marcus can join the Chosen One's party after the mine purifier has been fixed. He is skilled with big guns and large energy weapons. However, sometimes his use of burst weapons are as devastating to the party as to enemies. Also, as with all other super mutants (barring those in Fallout Tactics) he is incapable of wearing armor, which leaves him somewhat vulnerable. However, Marcus does have innate armor statistics that increase as he gains in abilities, thus mitigating his inability to equip armour. At his highest level, he has the equivalent of a suit of combat armor. Also, the ammunition for his big guns can be hard to come by and expensive to purchase (this can be dealt with, by arming Marcus with a turbo plasma rifle which, while still decent in terms of damage, has a much lower maintenance cost).
Having Marcus in your party will generally cause you to be denied entry into the inner sections of Vault City (though not the southern courtyard), so you will have to leave him outside and pick him up on the way out. Becoming Captain of the Guard allows you to get Marcus past the guards, but if First CitizenLynette sees you with him, you will be kicked out of the city for good.
Should you have both Lenny and Marcus in your party, Marcus will occasionally say things like "Nice to have another evolved human in the group, Lenny", "Hey Lenny! Ever been to Broken Hills?" and "Lenny! Let's get somethin' to drink!", indicating that the two have known each other in the past, or simply that Marcus considers their shared mutant status confers on them a brotherhood of sorts.
Taking him to Troy in Vault City to be healed will get him purged of the various bullets embedded in his thick skin, which Troy will then give to you. You get 20 7.62mm, 40 .44 magnum JHP, 40 .44 magnum FMJ, 50 5mm JHP, 10 .45 caliber, 24 10mm JHP, 50 .223 FMJ, and 20 9mm ball. He has been shot by a lot of people. Note that this can only be done if you are the Captain of the Guard, otherwise Troy will attack you for bringing a mutant into the city.
While Marcus says that he is no longer sterile, Chris Avellone, who wrote that dialogue, confirmed that Marcus was only joking and that super mutants remain entirely sterile.
During combat, if Marcus is hit by an enemy weapon, he may occasionally cry out, "I am NOT a merry Mutant!" This is a reference to Michael Dorn's role as Worf on Star Trek: The Next Generation, specifically the episode "Qpid", in which Worf protested to being given the role of a "merry man" in Q's Robin Hood recreation.
Marcus might say when unarmed and attacking: "one fist, two fist, red fist, blue fist". This is a reference to the Dr. Seuss book One Fish, Two Fish, Red Fish, Blue Fish.