From Las Vegas, Nevada, I-15 winds and slightly inclines through the Virgin River Gorge in northwest Arizona, then it crosses the border into Utah and Washington County.

Heading north-northeast, it continues with two lanes in each direction until it reaches an interchange with Southern Parkway (SR-7), which provides access to the St. George Regional Airport.

After SR-7, the route turns north as it passes the Bloomington area of the City of St. George and an interchange at Brigham Road, where an auxiliary lane is added before it crosses the Virgin River.

From this point north, I-15 generally follows the route of the Old US 91, maintaining a two-lane configuration until it reaches the Wasatch Front in the north-central part of the state.

An auxiliary lane is added between St. George Boulevard and Green Springs Road with additional interchanges at Washington Parkway and State Street (SR-9).

After passing through the northwestern part of Toquerville, and an interchange with SR-17 (at Anderson Junction), I-15 resumes a more north-northeastern course as it ascends about 1,000 feet (300 m) higher on the Colorado Plateau.

After Parowan, the freeway curves back to a northeast course as it passes about a mile (1.6 km) west of Paragonah and reaches an interchange on the north side of that city (SR-271).

Off the 1400 North interchange, there is an official rest stop, courtesy of Utah Department of Transportation (UDOT) and the local Texaco Station.

The freeway then heads slightly westward until it passes by the western side of the community of Manderfield and curves back to a north-northeast course.

SR-99 follows the length of the I-15 Business Loop through Fillmore, connecting at 850 South on the southwestern part of town and North Main Street / Cedar Mountain Road on the northern city limits.

Several miles northeast of Holden, US 50 merges with I-15 as it continues northeastward and eventually heading through a low mountain pass just after another ranch exit.

The freeway then continues roughly north-northeast until it turns northeast to pass through the southern part of Nephi and an interchange with South Main Street (SR-28).

Maintaining its course to the northeast, it passes the eastern edge of Benjamin, with an interchange with SR-164 (8000 South), and enters Spanish Fork and gains one lane in each direction.

The final interchange in Lehi is with Timpanogos Highway / Clubhouse Drive (SR-92), after which the Interstate gains an additional two lanes, which is just east of Thanksgiving Point.

After leaving Midvale, I-15 enters Murray and immediately reaches the junction with I-215, a beltway running through many of Salt Lake City's suburbs.

After entering Davis County, the city of North Salt Lake, and the Ogden–Clearfield metropolitan area, I-15 has the southbound on- and offramps for the Beck Street (US 89) interchange.

Curving slightly to the east, it passes by the western edge of Garland before coming to the Riverside (15200 North / SR-30) interchange, which is just southwest of that city and is the northern end of the I-15/SR-30 overlap.

Also, by the time it completed the section, UDOT recalibrated the mileposts and renumbered the exits, reducing the overall distance by approximately three miles (4.8 km).

The last section of I-15 within the state of Utah (as well as the entire length of the route) was finally completed in November 1990,[citation needed] only to have major reconstruction projects commence (starting in Salt Lake County) about 7 years later.

In September 2013, with a few brief exceptions, the speed limit north of Leeds (other than the Wasatch Front, between Santaquin and Brigham City) was raised to 80 mph (130 km/h).

[7] In 2015, the UDOT raised the speed limit on the Wasatch Front urban stretch through Salt Lake City to 70 mph (110 km/h).

In addition, a traffic management system was added to the entire length of the Interstate in the Salt Lake County region.

[13] The project was completed on December 15, 2012—35 months from the original notice to proceed—and finished $260 million under budget (coming in at $1.465 billion total).

[16] The two-year Point Project widened I-15 from four to six lanes in each direction between 12300 South in Draper and SR-92 in Lehi, a distance of approximately seven miles (11 km).

[17] The Utah Department of Transportation reconstructed the I-15 Technology Corridor between Lehi Main Street and SR-92 (Timpanogos Highway) starting in 2018.

Additional funding was recently added to the project that will widen 1800 North from I-15 to 2000 West, providing improved mobility to residents of Sunset and Davis County at large.

The study determined that a new interchange along with widening and providing a direct connection to 1500 West would improve safety and reduce congestion on mainline I-15 and US-89 by decreasing demand on Park Lane.

[22] In March 2022, the Utah Department of Transportation initiated an Environmental Impact Statement (EIS) for the Interstate 15 (I-15) Farmington to Salt Lake City Project.

The HOV lanes were available in both northbound and southbound directions spanning between 600 North in Salt Lake City and 10600 South in Sandy.

[24] Prior to completion of the South Davis Improvement project, there were no Express Lanes available between North Salt Lake and Farmington.