When wondering how Tom Hagen of The Godfather could be made consigliere the answer is simply that in real life, he could not. It might sound controversial to say this, but Sicilians back in that time were fiercely protective of who they brought into their inner circle within the mafia, and to think that a German-Irish individual would be accepted to such a role was simply unheard of, especially since Tom couldn’t even become a made man given that his lineage didn’t date back to the old country. But from a fictional standpoint, it did make sense considering that Don Vito had raised Tom for much of his life and had taken him in as a son, teaching him and making sure that Tom became a very trusted part of the family. Vito grew up having to earn everything he made, and he wanted different for his kids, and of course, since Tom was considered to be like one of his sons, he wanted him to learn as much as he could as well, meaning he wanted Tom to become a lawyer, to hone his intelligence, and to become someone that he could depend on for anything. The only thing is this, Tom was not a Sicilian as has already been mentioned, meaning he wasn’t born with and didn’t cultivate the burgeoning suspicion that the rest of the family, except maybe Sonny, had in abundance. In fact, it’s likely that Fredo didn’t have it either, since he was fairly simple and didn’t have a lot of ambition to do much other than to have fun with life. Connie had it, but it came much later after her naivete had finally worn off.
But Tom was also someone that would appeal to a lot of viewers since even decades ago people were still leery of each other, and had this movie featured only Italians it was believed that there wouldn’t have been enough of an opposing viewpoint that it would have done that well. In other words, Tom was thrown in as an anomaly and someone that could put a slightly different spin to the whole mob movie mentality. To be real, he wasn’t the best consigliere that was available since he did urge caution quite a bit, and was wise to do so when Sonny took over since the eldest Corleone sibling would have gladly started a war with the other families, which would have buried the Corleone’s. But the downfall of this was that Tom was all caution and very little action, and he was even ready to go in with Sollozzo on the drug trade since there was a lot of money to be made, but Don Vito nixed the deal, stating that it would be too much trouble. Tom Hagen wasn’t a bad man, and he wasn’t a horrible consigliere, but his compassion and his need for caution would have been the end of him at some point if not for the rest of his family since he wasn’t the type of guy to go to war without a very good reason. He had to plan things out, he needed a reason for everything, and that’s all well and good. But when Don Vito was in the hospital after a failed assassination attempt and Sonny was in charge, he needed to be way more decisive, and that just wasn’t who he was.
It wasn’t too much of a surprise when Michael removed Tom from the position of consigliere upon the Don’s decision to step down, but it was a pretty big slap in the face from someone that Tom had seen as a brother for so long. The truth of it though is that Tom couldn’t pull the trigger, figuratively speaking, when he had to. Plus, he wasn’t even able to pick out the traitor in the family since Vito and Michael were able to do this fairly easily. It could be said that Tom was just too trusting and not possessed of the same instincts that the others were, but it could also be said that as the peacemaker of the family his mind simply didn’t work that way. He was always looking for the way out of an argument or dispute and was trying to do things that would create the least blood since it was kind of obvious that Tom didn’t care for confrontations that much. But as a part of the family and someone that could be trusted to get things done when there was a plan in place, Tom was definitely one of those that could make things happen. But in real life, seeing anyone that was not a Sicilian as consigliere in the mafia would not happen as far as anyone is concerned, since not only were they worried about infiltration of their organization, but they were also fiercely loyal to their own bloodlines.