The heart of the computer is a Motorola 68000 microprocessor running at 7.8336 MHz, connected to 128 KB RAM shared by the processor and the display controller. The boot procedure and some operating system routines are contained in a 64 KB ROM chip. Apple did not offer RAM upgrades. Unlike the Apple II, no source code listings of the Macintosh system ROMs were offered.

The RAM in the Macintosh consists of sixteen 64k×1 DRAMs. The 68000 and video controller take turns accessing DRAM every four CPU cycles during display of the frame buffer, while the 68000 has unrestricted access to DRAM during vertical and horizontal blanking intervals. Such an arrangement reduces the overall CPU performance as much as 35% for most code as the display logic often blocks the CPU's access to RAM. Despite the nominally high clock rate, this causes the computer to run slower than several of its competitors and results in an effective clock rate of 6 MHz.

The built-in display is a one-bit per pixel, black-and-white, 9 in/23 cm CRT with a fixed resolution of 512 × 342 pixels, using the Apple standard of 72 ppi (pixels per inch), a standard that was quickly abandoned once higher resolution screens became available. Expansion and networking are achieved using two non-standard DE-9 serial ports named "Printer" and "Modem" that support the RS-422 standard, but do not support hardware handshaking. An external floppy disk drive can be added using a proprietary connector (19-pin D-sub).

The keyboard used a simple proprietary protocol, allowing some third-party upgrades. The mouse used standard quadrature signals for X and Y, and the single mouse button used a single wire (all signals were compatible with TTL and referenced to ground). The original keyboard had no arrow keys, numeric keypad or function keys. This was an intentional decision by Apple, as these keys were common on older platforms and it was thought that the addition of these keys would encourage software developers to simply port their existing applications to the Mac, rather than design new ones around the GUI paradigm. Later, Apple made a numeric keypad available for the Macintosh 128K. The keyboard sold with the newer Macintosh Plus model included the numeric keypad and arrow keys, but still no function keys. Function keys eventually appeared in 1987 with the Extended Keyboard available for the Macintosh II and Macintosh SE. As with the Apple Lisa before it, the mouse has a single button.

Standard headphones can be connected to a monaural jack on the back of the computer. Apple also offered their 300 and 1200 baud modems originally released for the Apple II line. Initially, the only printer available was the Apple ImageWriter, a dot matrix printer which was designed to produce 144 dpi WYSIWYG output from the Mac's 72 dpi screen. Eventually, the LaserWriter and other printers were capable of being connected using AppleTalk, Apple's built-in networking system.

The Macintosh contained a single 400 KB, single-sided 3+1⁄2-inch floppy disk drive, with no option to add any further internal storage, like a hard drive or additional floppy disk drive. The system software (Mac OS) was disk-based from the beginning, as RAM had to be conserved, but this "Startup Disk" could still be temporarily ejected. (Ejecting the root filesystem remained an unusual feature of the classic Mac OS until System 7.) One floppy disk was sufficient to store the System Software, an application and the data files created with the application.

The 400 KB drive capacity was larger than the PC XT's 360 KB 5.25-inch drive. However, more sophisticated work environments of the time required separate disks for documents and the system installation. Due to the memory constraints (128 KB) of the original Macintosh, and the fact that the floppies could hold only 400 KB, users had to frequently swap disks in and out of the floppy drive, which caused external floppy drives to be utilized more frequently. The Macintosh External Disk Drive (mechanically identical to the internal one, piggybacking on the same controller) was a popular add-on that cost US$495 (equivalent to $1,450 in 2023).

Third-party hard drives were considerably more expensive and usually connected to the slower serial port (as specified by Apple), although a few manufacturers chose to utilize the faster non-standard floppy port. The 128K can only use the original Macintosh File System released in 1984 for storage.