He became a carriage-maker at the age of sixteen, and opened his own carriage shop after five years.

He reinvested his fortunes in the San Francisco area, particularly in real estate, where he owned the Rancho Arroyo de la Alameda.

He started collecting rare books and art works, and also invested in securities and real estate.

He developed a deep interest in higher education; he visited and collected information about the universities in Europe.

[3] He started collecting a compact plot of land in the growing South End of Worcester in 1881.

On May 4, at the first regular meeting of the Board of Trustees, Clark disclosed his educational and financial plans for the new university.

[1] It opened with graduate departments in physics, chemistry, biology, mathematics, and psychology.

[5] Hall made multiple promises to the faculty of Clark University, but was unable to fulfill them.

Jonas Clark expressed his disappointment towards the affluent trustees, and to some extent towards the Worcester community, for not coming forward to support the university to Hall and the trustees, before leaving for Europe in 1891; he warned to reduce expenses if the public didn't give more support to the university.

From this point forward, Clark talked with Hall and the board particularly through letter; he also didn't spend much time in Worcester.

[1] The trustees didn't build the undergraduate college, something that Clark wanted, but they continued to appeal to him to financially support the university.

After his death, when the conditions of his will became known, the trustees discovered that they would have to establish an undergraduate college or else they would lose the remainder of Clark's fortune.