It most likely means dependable.
If you replace the word "solid" in the sentence with popular, unbroken, or immovable it doesn't makes sense like the word dependable does:
Mark had lived in a small city for all of his life, and local people thought of him as a popular citizen.
Mark had lived in a small city for all of his life, and local people thought of him as a unbroken citizen.
Mark had lived in a small city for all of his life, and local people thought of him as a immovable citizen.
Those don't make sense like:
Mark had lived in a small city for all of his life, and local people thought of him as a dependable citizen.
Hope this helped
