Otto Grün seems to fit the bill. Here's the abstract of From FLT to finite groups. The remarkable career of Otto Grün by Peter Roquette:

Every student who starts to learn group theory will soon be confronted with the theorems of Grün. Immediately after their publication in the mid 1930s these theorems found their way into group theory textbooks, with the comment that those theorems are of fundamental importance in connection with the classical Sylow theorems. But little is known about the mathematician whose name is connected with those theorems. In the following we shall report about the remarkable mathematical career of Otto Grün who, as an amateur mathematician without having had the opportunity to attend university, published his first paper (out of 26) when he was 46. The results of that first paper belong to the realm of Fermat’s Last Theorem (abbreviated: FLT). Later Gr¨un switched to group theory.

(emphasis by me)

I ran into this article through this answer to this question on MO.

Otto Grün seems to fit the bill. Here's the abstract of From FLT to finite groups. The remarkable career of Otto Grün by Peter Roquette:

Every student who starts to learn group theory will soon be confronted with the theorems of Grün. Immediately after their publication in the mid 1930s these theorems found their way into group theory textbooks, with the comment that those theorems are of fundamental importance in connection with the classical Sylow theorems. But little is known about the mathematician whose name is connected with those theorems. In the following we shall report about the remarkable mathematical career of Otto Grün who, as an amateur mathematician without having had the opportunity to attend university, published his first paper (out of 26) when he was 46. The results of that first paper belong to the realm of Fermat’s Last Theorem (abbreviated: FLT). Later Gr¨un switched to group theory.

(emphasis by me)

I ran into this article through this answer to this question on MO.

Otto Grün seems to fit the bill. Here's an excerpt of the abstract of From FLT to finite groups. The remarkable career of Otto Grün by Peter Roquette:

... We shall report about the remarkable mathematical career of Otto Grün who, as an amateur mathematician without having had the opportunity to attend university, published his first paper (out of 26) when he was 46.

I ran into this article through this answer to this question on MO.

Otto Grün seems to fit the bill. Here's the abstract of From FLT to finite groups. The remarkable career of Otto Grün by Peter Roquette:

Every student who starts to learn group theory will soon be confronted with the theorems of Grün. Immediately after their publication in the mid 1930s these theorems found their way into group theory textbooks, with the comment that those theorems are of fundamental importance in connection with the classical Sylow theorems. But little is known about the mathematician whose name is connected with those theorems. In the following we shall report about the remarkable mathematical career of Otto Grün who, as an amateur mathematician without having had the opportunity to attend university, published his first paper (out of 26) when he was 46. The results of that first paper belong to the realm of Fermat’s Last Theorem (abbreviated: FLT). Later Gr¨un switched to group theory.

(emphasis by me)

I ran into this article through this answer to this question on MO.