Hank Aaron has hit the most career home runs by a player younger than 42, with 745 homers.

Interpreted as:

most homerun before turning 42 years old by a player

	NAME	HR	G	AB	R	H	2B	3B	RBI	BB	HBP	SO	SB	CS	PA	TB	XBH	SH	SF	IBB	GIDP	AVG	OBP	SLG	OPS
1	Hank Aaron H. Aaron	745	3,213	12,093	2,153	3,709	616	98	2,262	1,368	32	1,343	240	72	13,633	6,756	1,459	21	119	292	322	.307	.375	.559	.934
2	Barry Bonds B. Bonds	722	2,809	9,352	2,124	2,796	576	77	1,897	2,394	101	1,467	509	141	11,940	5,692	1,375	4	89	639	148	.299	.443	.609	1.052
3	Babe Ruth B. Ruth	714	2,504	8,400	2,173	2,873	506	136	2,213	2,062	44	1,331	123		10,622	5,793	1,356	116				.342	.474	.690	1.164
4	Alex Rodriguez A. Rodriguez	696	2,784	10,566	2,021	3,115	548	31	2,086	1,338	176	2,287	329	76	12,207	5,813	1,275	16	111	97	261	.295	.380	.550	.930
5	Albert Pujols A. Pujols	679	2,971	11,114	1,872	3,301	672	16	2,150	1,345	113	1,349	116	41	12,690	6,042	1,367	1	117	315	413	.297	.375	.544	.919
6	Willie Mays W. Mays	654	2,937	10,709	2,039	3,243	514	140	1,879	1,445	43	1,489	337	102	12,299	5,999	1,308	12	90	193	248	.303	.385	.560	.945
7	Ken Griffey Jr. K. Griffey Jr.	630	2,671	9,801	1,662	2,781	524	38	1,836	1,312	81	1,779	184	69	11,304	5,271	1,192	8	102	246	199	.284	.370	.538	.907
8	Jim Thome J. Thome	611	2,533	8,390	1,581	2,320	449	26	1,695	1,743	69	2,534	19	20	10,277	4,654	1,086	1	74	172	163	.277	.402	.555	.957
9	Sammy Sosa S. Sosa	609	2,354	8,813	1,475	2,408	379	45	1,667	929	59	2,306	234	107	9,896	4,704	1,033	17	78	154	202	.273	.344	.534	.878
10	Frank Robinson F. Robinson	586	2,808	10,006	1,830	2,943	528	72	1,812	1,418	198	1,532	203	79	11,741	5,373	1,186	17	102	218	279	.294	.389	.537	.926
11	Mark McGwire M. McGwire	583	1,874	6,187	1,167	1,626	252	6	1,414	1,317	75	1,596	12	8	7,660	3,639	841	3	78	150	147	.263	.394	.588	.982
12	Harmon Killebrew H. Killebrew	573	2,435	8,147	1,282	2,086	290	24	1,583	1,561	48	1,704	19	20	9,833	4,143	887	0	77	161	243	.256	.376	.509	.884
13	Rafael Palmeiro R. Palmeiro	569	2,831	10,472	1,663	3,020	585	38	1,834	1,353	87	1,348	97	40	12,046	5,388	1,192	15	119	172	232	.288	.371	.515	.885
14	Reggie Jackson R. Jackson	563	2,820	9,864	1,551	2,584	463	49	1,702	1,376	96	2,596	228	115	11,417	4,834	1,075	13	68	164	183	.262	.356	.490	.846
15	Manny Ramirez M. Ramirez	555	2,302	8,244	1,544	2,574	547	20	1,831	1,329	109	1,813	38	33	9,774	4,826	1,122	2	90	216	243	.312	.411	.585	.996
16	Mike Schmidt M. Schmidt	548	2,404	8,352	1,506	2,234	408	59	1,595	1,507	79	1,883	174	91	10,062	4,404	1,015	16	108	201	156	.267	.380	.527	.908
17	David Ortiz D. Ortiz	541	2,408	8,640	1,419	2,472	632	19	1,768	1,319	38	1,750	17	9	10,091	4,765	1,192	2	92	209	236	.286	.380	.552	.931
18	Mickey Mantle M. Mantle	536	2,401	8,101	1,675	2,415	344	72	1,509	1,733	12	1,718	152	42	9,907	4,511	952	14	47	126	113	.298	.420	.557	.977
19	Jimmie Foxx J. Foxx	534	2,317	8,134	1,751	2,645	458	125	1,924	1,457	13	1,311	87		9,676	4,955	1,117	72			70	.325	.428	.609	1.038
20	Frank Thomas F. Thomas	521	2,322	8,199	1,494	2,468	495	12	1,704	1,667	87	1,397	32	23	10,074	4,550	1,028	0	121	168	226	.301	.419	.555	.974
21	Willie McCovey W. McCovey	520	2,540	8,084	1,221	2,188	345	46	1,540	1,333	68	1,527	26	21	9,557	4,185	911	5	67	258	176	.271	.376	.518	.893
22	Ted Williams T. Williams	516	2,266	7,640	1,788	2,635	521	71	1,826	2,002	39	700	24	4	9,706	4,846	1,108	5	20	88	200	.345	.482	.634	1.116
23	Ernie Banks E. Banks	512	2,528	9,420	1,306	2,584	407	90	1,636	763	70	1,234	49	54	10,394	4,707	1,009	45	96	197	231	.274	.330	.500	.830
24	Eddie Mathews E. Mathews	512	2,391	8,537	1,509	2,315	354	72	1,454	1,443	26	1,490	69	41	10,100	4,349	938	36	58	107	127	.271	.376	.509	.885
25	Miguel Cabrera M. Cabrera	511	2,797	10,356	1,551	3,174	627	17	1,881	1,258	74	2,105	40	21	11,796	5,368	1,155	5	103	238	364	.306	.382	.518	.901