After spending four years in the United States Army, he transformed the Viking Brotherhood into the Asatrú Free Assembly (AFA), through which he promoted Heathenry within the American Pagan community.

He espoused the belief, which he named "metagenetics", that religions are connected to genetic inheritance, thus arguing that Heathenry was only suitable for those of Northern European ancestry.

A growing membership generated internal conflict within the AFA, resulting in McNallen's decision to expel those with neo-Nazi and racial extremist views from the organisation.

Moving to Northern California, McNallen began a career as a school teacher; during the summer vacations he travelled the world as a military journalist, writing articles for Soldier of Fortune magazine.

Concerned by what he saw as the growth of liberal, universalist ideas in Heathenry, he returned to active involvement in the Heathen movement in the mid-1990s, establishing the Asatrú Folk Assembly, which was headquartered in Grass Valley, California.

He brought greater attention to his group after they became involved in the debate surrounding the Kennewick Man, arguing that it constituted evidence for a European presence in prehistoric America.

His espousal of right-wing ethnonationalist ideas and his insistence that Heathenry should be reserved for those of Northern European ancestry has resulted in accusations of racism from both Pagans and the mainstream media.

Conversely, many on the extreme right of the Heathen movement have accused him of being a race traitor for his opposition to neo-Nazism and refusal to endorse white supremacism.

[6] In the winter of 1971–72 he began publishing a newsletter, The Runestone, using a typewriter and mimeograph machine; he gained his first eleven subscribers through an advert that he placed in Fate magazine.

[12] He then changed it once again, this time to "Asatru", which he had discovered in Magnus Magnusson's book, Hammer of the North, and subsequently popularized within the American Heathen community.

[14] After completing his college education he joined the United States Army, remaining with them for four years, volunteering for service in the Vietnam War before being stationed in West Germany.

[12] He retained his interest in Heathenry while a member of the army, and circa 1974 adopted the belief that there was an intrinsic connection between the Norse gods and humans of Northern European descent.

[16] "What makes metagenetics so difficult for many to accept is its base assumption: the tradition (i.e., culture) is a matter of genetic inheritance, and it is for this reason that the compulsion to reawaken the Northern Way has come upon some but not others.

[12] The sociologist of religion Jennifer Snook described it as "the first national Heathen organization in the United States",[18] while according to the religious studies scholars Michael F. Strmiska and Baldur A. Sigurvinsson, the AFA "established many of the important organizational and ritual structures that remain operative" in American Heathenry into the 21st century.

[36] As a result of such changes, by the late 1970s there was a clear division between McNallen's AFA, which emphasized religious over racial political aims, and Christensen's Odinist Fellowship, which placed far greater emphasis on the latter.

While the autonomous nature of different AFA-affiliated individuals and groups meant that McNallen and his wife Maddy Hutter had little power, they bore the brunt of the responsibility of running the Assembly and organizing its Althings.

[44][2] During his summer vacations he travelled abroad,[44] during which he met with guerrilla groups active in various parts of the world, writing articles about them for magazines like Soldier of Fortune.

[49] This group based its headquarters in Grass Valley,[21] and was structurally very similar to the old AFA,[50] reviving McNallen's The Runestone publication, albeit in a yearbook format.

[51] The AFA served much the same constituency as the established AA, with religious studies scholar Jeffrey Kaplan believing that its purpose in the Heathen community was therefore largely superfluous.

[62] During the late 1990s, the AFA began promoting what it termed "tribalism", encouraging Heathens to form networks of extended families to help secure the religion's future.

[56] Around the same time it began to promote the writings of Nouvelle Droite authors such as Alain de Benoist, selling an English-language translation of the latter's Being a Pagan on its website.

[74] In 2011, the AFA sent a contingent to the annual conference of a Spencer's white nationalist organization, the National Policy Institute, in what some figures close to McNallen revealed was an attempt to recruit members.

[84] In 2018, the Southern Poverty Law Center added the Asatru Folk Assembly to its list of hate groups, describing it as a "Neo-völkisch hate group"[85] In October 2023, Stephen McNallen gave a speech about "Wotan's dual aspects of fury and wisdom, as noted by Dr. Carl Jung" and his recent book "The Spear"[86] at the American Renaissance conference hosted by AmRen who makes the claim to be "America's foremost white advocacy publication.

[93] He regards Heathenry as belonging to all people descended from ancient northern Europeans and wants to make it as accessible as possible to this demographic, arguing that it is the religion best suited to fill the spiritual yearning of many white Americans.

[91] He also ties this idea in with concepts drawn from Jungian psychology, arguing that archetypes are distinct to specific racial groups and are passed down genetically.

"[96] According to a 1998 Southern Poverty Law Center article, although McNallen has attempted to evade the "Nazi-Odinist identification", he has "expressed sympathy with what he sees as the "legitimate frustrations of white men who are concerned for their kind".

[97] "The spiritual descendants of the Aztecs are looking northward, coveting land which, they have convinced themselves, should be theirs–and, perhaps quite unconsciously, they are moving to conquer it by mass immigration, by language, by cultural influence.

"[101] According to Snook, McNallen's concerns reflect "white American resentment of a perceived loss of status and privilege in modern, multicultural society in particularly divisive political times.

"[112] However, in writings and YouTube videos, Stephen McNallen has spoken favorably of Wotanism and the Fourteen Words of David Lane, networking with Red Ice TV, a social media outlet of the alt-right.

[114] Calico thought that "McNallen deserves to be noted as one of many American religious dissenters who, because of strong personal conviction and vision, stood their ground in the face of hardship and opposition.