No. Mandatory means that the USSC has held that certain kinds of federal spending are promises by the government to provide certain goods and services directly to citizens, and MUST be made regardless of budget situation, as they are a contractual promise of the government. Congress and the President can do very little to affect this in the short term. For the most part mandatory spending comes from the trust funds, such as social security and medicare, for which there are specific taxes generating funds to pay for these goods and services.
Discretionary spending means that the court has ruled that spending on these items are cuttable, including such things as national defense and many other programs, as they are not viewed as contractual promises to the people, and Congress can approve whatever budget and appropriations bills it wants (it usually largely or completely rejects the President's proposed budget, and builds its own), and the Administration can change some of that. These "federal funds" come from the income (individual and corporate) and a number of other taxes and fees and are also referred to as "general revenues."
All revenue and spending for both kinds is "on budget." The Bush/Cheney promise that the Gulf War was going to be "off budget" was nonsense that they should have been called on from the word go--I remember that some did, but in the aftermath of 9-11, very few people seemed to be interested. The SS and Medicate and other trust fund taxes for the most part have resulted in surpluses that are meant to cover future spending...but of course, Congress has repeatedly raided--"borrowed"--from these funds over the years, and for the most part has not paid it back. On the other hand, in most years, Congress spends more on the rest of the programs than it takes in in taxes. THAT is where most of the deficit spending--and therefore the debt--comes from.
