After he was returned to Rome, he spent his time for the most part either at home, in conversation with Athenodorus, or at the forum, in the service of his friends. Though it was now the time that he should become quaestor, he would not stand for the place till he had studied the laws relating to it, and by inquiry from persons of experience, had attained a distinct understanding of the duty and authority belonging to it. With this knowledge, as soon as he came into the office, he made a great reformation among the clerks and under-officers of the treasury, people who had long practice and familiarity in all the public records and the laws, and, when new magistrates came in year by year so ignorant and unskillful as to be in absolute need of others to teach them what to do, did not submit and give way, but kept the power in their own hands, and were in effect the treasurers themselves. Till Cato, applying himself roundly to the work, showed that he possessed not only the title and honour of a quaestor, but the knowledge and understanding and full authority of his office. So that he used the clerks and under-officers like servants as they were, exposing their corrupt practices, and instructing their ignorance. Being bold, impudent fellows, they flattered the other quaestors his colleagues, and by their means endeavoured to maintain an opposition against him. But he convicted the chiefest of them of a breach of trust in the charge of an inheritance, and turned him out of his place. A second he brought to trial for dishonesty, who was defended by Lutatius Catulus, at that time censor, a man very considerable for his office, but yet more for his character, as he was eminent above all the Romans of that age for his reputed wisdom integrity. He was also intimate with Cato, and much commended his way of living. So perceiving he could not bring off his client, if he stood a fair trial, he openly began to beg him off. Cato objected to his doing this. And when he continued still to be importunate, "It would be shameful, Catulus," he said, "that the censor, the judge of all our lives, should incur the dishonour of removal by our officers." At this expression, Catulus looked as if he would have made some answer; but he said nothing and either through anger or shame went away silent, and out of countenance. Nevertheless, the man was not found guilty, for the voices that acquitted him were but one in number less than those that condemned him, and Marcus Lollius, one of Cato's colleagues, who was absent by reason of sickness, was sent for by Catulus, and entreated to come and save the man. So Lollius was brought into court in a chair, and gave his voice also for acquitting him. Yet Cato never after made use of that clerk, and never paid him his salary, nor would he make any account of the vote given by Lollius. Having thus humbled the clerks, and brought them to be at command, he made use of the books and registers as he thought fit, and in a little while gained the treasury a higher name than the senate-house itself; and all men said, Cato had made the office of a quaestor equal to the dignity of a consul. When he found many indebted to the state upon old accounts, and the state also in debt to many private persons, he took care that the public might no longer either do or suffer wrong;he strictly and punctually exacted what was due to the treasury, and as freely and speedily paid all those to whom it was indebted. So that the people were filled with sentiments of awe and respect, on seeing those made to pay, who thought to have escaped with their plunder, and others receiving all their due, who despaired of getting anything. And whereas usually those who brought false bills and pretended orders of the senate, could through favour get them accepted, Cato would never be so imposed upon; and in the case of one particular order, on the question arising whether it had passed the senate, he would not believe a great many witnesses that attested it, nor would admit of it, till the consuls came and affirmed it upon oath.
There were at that time a great many whom Sylla had made use of as his agents in the proscription, and to whom he had for their service in putting men to death, given twelve thousand drachmas apiece.
These men everybody hated as wicked and polluted wretches, but nobody durst be revenged upon them. Cato called every one to account, as wrongfully possessed of the public money, and exacted it of them, and at the same time sharply reproved them for their unlawful and impious actions. After these proceedings they were presently accused of murder, and being already in a manner prejudged as guilty, they were easily found so, and accordingly suffered; at which the whole people rejoiced and thought themselves now to see the old tyranny finally abolished, and Sylla himself, so to say, brought to punishment.