In writing a comparative essay about the two works, you might like to concentrate on what they say about the economic and political situation in 1930s America.

Both books are set during the height of the Great Depression, when poverty and mass unemployment stalked the land. For itinerant farm laborers like Lennie and George in Of Mice and Men, this means that they're forced to travel from place to place in search of work. As unemployment is so high, ranch owners have the whip hand over their employees. They can pretty much treat their workers as they please, as they know that there will always be many others willing to take their place.

In To Kill a Mockingbird it's clear that the rural areas surrounding Maycomb have been especially hard hit by the Depression. We see this illustrated in the wretched poverty of the Cunningham family, who pay in kind for services rendered—such as Atticus's legal advice—as they can't afford to pay in cash.

Another point of comparison is the issue of race. Although Of Mice and Men takes place in California and not the Deep South, we soon discover that this part of the world is no less prone to racial bigotry and prejudice than Maycomb.

It says a lot about racial relations in 1930s America that Crooks, the only black man working on the ranch, is segregated from the other men, forced to live apart from them in his own cramped little room. He's also threatened with being lynched by Curley's wife for not treating her with the respect she feels is her due as a white woman.

A more brutal demonstration of this power dynamic arises in To Kill a Mockingbird, where Tom Robinson is convicted on a trumped-up charge of assaulting and raping a white woman, Mayella Ewell.

Both To Kill a Mockingbird and Of Mice and Men are works of literary excellence to ensure both literary works an enduring place in modern American literature. 

To compare To Kill A Mockingbird and Of Mice and Men, the reader can see the racial inequality of America during the 1930s. No doubt, black people are treated unfairly in the 1930s. There is discrimination in each novel.

In To Kill A Mockingbird, Tom Robinson is falsely accused of raping a white woman. Clearly, Atticus proves he is not guilty, but due to an all-white racial jury, Tom is found guilty of a crime he did not commit. Truly, Lee's novel is based on instances of the 1930s. There was racial inequality in America. Tom Robinson did not stand a chance in the racist America of the 1930s. 

The racially divided town and the strict class system help the reader to visualize life in the South during this time period.

Of Mice and Men was also filled with racial inequality in America in the 1930s. Crooks was the only black ranch hand, and he was treated unfairly. Crooks was not invited to share in the activities that the other white ranch hands enjoyed. He was ostracized. He had to sleep in different quarters than the white ranch hands. He slept in the harness room away from the white ranch hands. Crooks knew he was disliked because of the color of his skin. He did not receive equal treatment because he was black. While talking with Lennie, Crooks is obviously upset about the discrimination he faced:

Crooks said gently, “Maybe you can see now. You got George. You know he’s goin’ to come back. S’pose you didn’t have nobody. S’pose you couldn’t go into the bunk house and play rummy ‘cause you was black. How’d you like that? S’pose you had to sit out here an’ read books. Sure you could play horseshoes till it got dark, but then you got to read books. Books ain’t no good. A guy needs somebody—to be near him.” He whined, “A guy goes nuts if he ain’t got nobody. Don’t make no difference who the guy is, long’s he’s with you. I tell ya,” he cried, “I tell ya a guy gets too lonely an’ he gets sick.”

Clearly, Crooks lives a lonely existence. He is judged based on the color of his skin. Because he is black, he is considered inferior to white people.

Likewise, Tom Robinson is judged by the color of his skin. He is considered guilty of crossing racial barriers. He is automatically found guilty a a crime he did not commit only because he is black.

Truly, To Kill A Mockingbird and Of Mice and Men are set in the time of racial discrimination in America during the 1930s. Both are considered literary classics. Both literary works have historical significance at a time when America was highly racist. The reader can learn so much about the extremely racist period of the 1930s in America by reading both literary classics.