How can I get input radio elements to horizontally align in react [material-ui]?

Simply use the row property:

<RadioGroup row><Radio /><Radio /></RadioGroup>

RadioGroup inherits from FormGroup so the properties of the FormGroup component are also available.

Another example, with labels:

<RadioGroup aria-label="anonymous" name="anonymous" value={false} row>
  <FormControlLabel value="true" control={<Radio />} label="Yes" />
  <FormControlLabel value="false" control={<Radio />} label="No" />
</RadioGroup>