I'm trying to say: throughout someone's life, hard work was a constant theme and I was going to use:"his whole life was underscored by hard work" but I'm pretty sure that's not what that means at all and it might not even make sense. Does anyone know what phrase I'm looking for?
You actually said it: A constant.
This phasing is common to describe a quality that doesn't change.
Throughout his life, despite changes in circumstances, hard work was a constant.
Throughout his life, the only constant was his dedication to hard work.
The question might be, was this character forced to work hard, eager to work hard, stuck working hard? What is hard work? Is it the effort you put into it, or unfortunate circumstances, such as terrible conditions, horrible bosses, physical labor? Just saying "hard work was a constant" in someone's life does not give much insight into either the work or the person.
I'd use keynote.
- a prevailing tone or central theme ....
- Individuality is the keynote of the Nineties.
'Hard work was the keynote of his life.'
One could also use 'Hard work was the defining feature of his life' or 'his life was defined by hard work'.
Hard work was the cornerstone of his entire life.
A basic element : FOUNDATION
A cornerstone of foreign policy m-w
The cornerstone of something is the basic part of it on which its existence, success, or truth depends. Collins
Something that is essential, indispensable, or basic.
The cornerstone of democratic government is a free press. dictionary.com
... but "despite all the motley features of Peter's character, his was a surprisingly whole nature. The idea of serving the state, in which the tsar profoundly believed and to which he subordinated his activities, was the cornerstone of his life." P. P. Pavlenko, quoted in D. J. Raleigh and A. A. Iskenderov; The Emperors and Empresses of Russia
The city was the sphere of both morality and politics and Socrates' commitment to improving his fellow citizens was the cornerstone of his life... Vijay Tankha; Ancient Greek Philosophy
You could say
Hard work was the leitmotif of his whole life.
Leitmotif is used here metaphorically:
A leitmotif in something such as a book or film or in a person's life is an idea or an object which occurs again and again. (Collins)
- There is no doubt that the system will need reform, and that reform will be the leitmotiv of politics over the next decade. (From the Hansard archive - Cambridge)
M-W notes about leitmotif:
Leitmotif is still commonly used with reference to music and musical drama but is now also used more broadly to refer to any recurring theme in the arts or in everyday life.
- His passion for running was the leitmotif of his life. (New York Times)